I could honestly not make much sense of the timer implementation
that was here. Instead I've implemented the type of timer algorithm
that I've used before, and tested it instead.

The concept is extremely simple: all timers are put in an ordered
list. We check every server tick if any of the timers have
elapsed, and execute the function associated with this timer.

We know that many timers by themselves cause new timers to be
added to this list, so we iterate *backwards* over the timer
list. This means that new timers being added while timers are
being executed, can never be executed in the same function pass,
as they are always appended to the table *after* the end of
the table, which we will never reach in the current pass over
all the table elements.

We switch time keeping to minetest.get_us_time(). dtime is
likely unreliable and we have our own high-res timer that we
can fix if it is indeed broken. This removes the need to do
any sort of time keeping.

clock_gettime() is a far better clock than gettimeofday().

Even better than clock_gettime() is that you can select either
CLOCK_MONOTONIC, or even CLOCK_MONOTONIC_RAW. These clocks offer
high precision time. And the _RAW variant will never roll back
due to NTP drift or daylight savings, or otherwise.

I've adjusted this code to select the right clock method auto-
matically based on what's available in the OS. This means that
if you're running a very old linux version, MacOS or other,
you will automatically get the best clocksource available.

I've tested all Linux clocksources by selectively compiling and
running a 10k+ timer test suite. In all cases I confirmed that
the 3 POSIX Linux clocksources worked properly, and were
selected properly.

I've modified the OS X compile path to use the high-res clock
source for all time functions, but I can't confirm it works or
that it compiles.

As for WIN32, I confirmed that the used clocksource is indeed
a Monotonic clocksource, so good news: that code section appears
to be exactly what it should be.

Backface culling is enabled by default for all tiles, as this
is how the lua parser initializes each tiledef. We revert to
always using the value from the tiledef since it is always
read and serialized.

Mods that wish to enable culling for e.g. mesh nodes, now can
specify the following to enable backface culling:

    tiles = {{ name = "tex.png", backface_culling = true }},

Note the double '{' and use of 'name' key here! In the same
fashion, backface_culling can be disabled for any node now.

I've tested this against the new door models and this properly
allows me to disable culling per node. I've also tested this
against my crops mod which uses mesh nodes where culling needs
to be disabled, and tested also with plantlike drawtype nodes
where we want this to continue to be disabled.

No default setting has changed. The defaults are just migrated
from nodedef.cpp to c_content.cpp.

corrected this bit reflect the function properly.

Lua_api.txt: Document 'minetest.registered_biomes'
Minimal: Remove 'mapgen_air' alias
Cavegen: Add fallback node for 'mapgen_ice'
Dungeongen: Add fallback node for 'mapgen_river_water_source'
Mgv5: Remove unnecessary '#include util/directiontables.h'
Add missing 'this->'s in makeChunk()
Mgv6: Edit empty line formatting
Remove leading spaces in makeChunk()
Add missing spaces after 'for' and 'if'
Mgv7: Edit empty line formatting

I apparently forgot to add this file in my previous commit (bd40ee2b).

Fixed spelling mistake!

set_sky does work with on_joinplayer

Add new node property 'floodable', default false
Define "air" as floodable = true in C++ and lua

* Add main page.
* Organize into sections.
* Add threading sources.
* Include SpatialAreaStore, LevelDB/Redis, sound, FreeType, and cURL in output.
* Add logo.
* Fix project name hardcoding.
* Remove PAPER_TYPE (only used when GENERATE_LATEX is enabled).
* Have dot render graphs as SVG (smaller, and works even if dot's text drawing functionality is broken).
* Enable built-in STL support.
* Enable search bar.
* Switch from header-bar based navigation to treeview based navigation.
* Enable dynamic HTML (collapses graphs).
* Enable generation timestamp.

or change the nametag text of players

If rendered as markdown, lists need a blank line before them
so that they are recognized as such.

To terminate unwanted shadows from floatlands or realms above
Also add to LuaVoxelManip calc_lighting for use in mapgen mods
Remove the 2 argument calcLighting, mapgens now use the 5
argument form to specify the volumes for propagateSunlight and
spreadLight
In mgsinglenode replace calcLighting with setLighting and
clean-up use of tabs and spaces

This adds a chat console the server owner can use for administration
or to talk with players.
It runs in its own thread, which makes the user interface immune to
the server's lag, behaving just like a client, except timeout.
As it uses the same console code as the f10 console, things like nick
completion or a scroll buffer basically come for free.
The terminal itself is written in a general way so that adding a
client version later on is just about implementing an interface.

Fatal errors are printed after the console exists and the ncurses
terminal buffer gets cleaned up with endwin(), so that the error still
remains visible.

The server owner can chose their username their entered text will
have in chat and where players can send PMs to.
Once the username is secured with a password to prevent anybody to
take over the server, the owner can execute admin tasks over the
console.

This change includes a contribution by @kahrl who has improved ncurses
library detection.

Fix memory leak in minetest.place_schematic
Slightly refactor Schematic code

- Disabled by default (except players)
- Fixes #2984

* Add key settings to setting table and ignore them later
  This way they are added to the auto-generated minetest.conf.example
* Add flags type
* Add input validation for int, float and flags
* Break in-game graphic settings into multiple sections
* Parse settingtpes.txt in mods and games
* Improve description for a lot of settings
* Fix typos and wording in settingtypes.txt
* Convert language setting to an enum

Add findLiquidSurface() function to mapgen.cpp
Update lua_api.txt

The callback can now be invoked with either the player object or name as
the first parameter, and with either a table or a list of strings, like
this:

    minetest.check_player_privs(player_name, { shout = true, fly = true })
    minetest.check_player_privs(player_name, "shout", "fly")
    minetest.check_player_privs(player, { shout = true, fly = true })
    minetest.check_player_privs(player, "shout", "fly")

Default is true for backwards compatibility
Update lua_api.txt

- Add warning log level
- Change debug_log_level setting to enumeration string
- Map Irrlicht log events to MT log events
- Encapsulate log_* functions and global variables into a class, Logger
- Unify dstream with standard logging mechanism
- Unify core.debug() with standard core.log() script API

Previous statements were wrong.

This might break some mods, but it is important for all uses of the param2 to
be documented.

This doesn't need a serialisation version or network protocol version change,
as old clients will still work on new servers, and it is bearable to have
new clients getting non rotated plants on old servers.

It returns the index used in mg->biomemap for a given biome name.
The biomemap is useless without this unless you re-register all existing biomes,
which could cause problems for anyone else trying to use biomemap.
With this, you can quickly create a lookup table of ids and names.

Because not all circles are round:
* circles using an euclidean metric are what we usually call "round"
* circles using a maximum metric look like euclidean rectangles with equal adjacent sides (squares)
* circles using a manhattan metric look like an euclidean right angled rhombus (squares, but 45° rotated to the former one)

[ci skip]