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]

Thanks to @kaadmy (NeD) for pointing this out.

Document the node metadata changes of the commit (serialisation version >=23):

704782c9 "WIP node metadata, node timers"

Fix regression since commit:

915807f8 "Rename doc/mapformat.txt and update doc to match SRP changes"

And add an underscore to doc file name to make it more readable.

1. Do two renames:
	* SER_FMT_CLIENT_VER_LOWEST -> SER_FMT_VER_LOWEST_WRITE
	* SER_FMT_VER_LOWEST -> SER_FMT_VER_LOWEST_READ
Now the two define values are consistently named with the _WRITE defines
SER_FMT_VER_{HIGHEST,LOWEST}_WRITE, and to better point out what the two
serialisation versions actually are for.

2. wrap some lines in doc/worldformat.txt, and point out that the node
timers are serialized at a later point, as this can cause confusion about
what now happens (if one doesn't strictly read the if block's conditions).

3. some whitespace fixes in NodeTimerList::serialize, and one new comment.

The documentation file contains not just information about the
map itself, but also about further files inside the world's
directory.

Documentation didn't reflect recent SRP addition, now it does.

Modders are now able to select the range of ore column height,
and the midpoint at which they 'grow' starting from.
This commit adds three new parameters for the 'sheet' ore type:
column_height_min, column_height_max, and column_midpoint_factor.
clust_size is now deprecated for this ore type.

Enable the server to request the client to reconnect.

This can be done with the now extended minetest.request_shutdown([reason], [reconnect]) setting.

The neighbours checked are the 8 nodes horizontally surrounding the decoration base
and the 8 nodes horizontally surrounding the ground node below the decoration

Adds list-rings, a method to implement item sending between inventories via shift-click.
Nice insider feature: a ring consisting of a single inventory list serves as nice clean-up method.
Also adds them to minimal game, and the standard inventory.
Craft output slots are not supported.