Undoubtably this may cause problems later if unchecked.

```
2016-03-22 21:57:52: WARNING[Server]: Assignment to undeclared global "options" inside a function at .../sofar/git/minetest/bin/../builtin/game/chatcommands.lua:862.
```

ABM's are hardcoded to run every 1.0s, NodeTimers are hard coded to
run at every 1.0s. Block mgmt is running every 2.0sec.

However, these timers can be better tuned for both higher and lower
values by server owners. Some server owners want to, and have the
resources to send more packets per second to clients, and so they
may wish to send smaller updates sooner. Right now all ABM's are
coalesced into 1.0 second intervals, resulting in large send queues
to all clients. By reducing the amount of possible timers, one can
get a far better response rate and lower the perception of lag.

On the other side of the camp, some servers may want to increase
these values, which again isn't easily doable.

The global settings abm_interval and nodetimer_interval are set to
current values by default. I've tested with 0.2/0.5 type values
and noticed a greatly improved response and better scattering of
nodetimers, as well as enjoying not faceplanting into doors with
pressure plates anymore.

The legacy init packet (pre v25) sends information about the client's
password that a server could use to log in to other servers if the
username and password are the same. All the other benefits of SRP of
protocol v25 are missed if the legacy init packet is still sent during
connection creation.

This patch adds an option to not send the v25 init packet. Not sending
the v25 packet means breaking compat with pre v25 servers, but as the
option is not enabled by default, no servers are affected unless the
user explicitly flips the switch. More than 90% of the servers on the
serverlist support post v25 protocols.

The patch also fixes a bug with greying out of non compliant servers
being done wrongly, the min and max params were mixed.

This privilege allows map protection bypassing for server operators
and world moderators.

Initially I had thought that bypassing protection mods would have been
something that could entirely be done inside mods and minetest_game,
but the concept of protection is defined in core, in the code of
core.is_protected().

I don't feel that it would be logical to introduce a protection
concept in core, but not some way around that for server operators
to maintain map parts that need fixing, de-griefing or cleanup.

Others had noticed the same problems, and proposed a patch to
minetest_game. That patch is fine by itself, but it fails to add
protection bypass functionality for digging normal nodes and placing
nodes.

So, instead, we indroduce the new priv "protection_bypass" in core,
and modify 'on_place_node' and 'node_dig' to allow bypassing node
protections if the player holds this priv.

This priv was tested with protector redo by tenplus1.

A followup patch to Minetest Game will include allowing special checks
for doors, trapdoors, chests in Minetest Game.

Protection mods will likely want to mimic the changes in their relevant
code sections.

I've written several experimental bits of code that revolve around the
need for a consistent calendar, but implementing one is extremely hard
in mods due to time changes and mods overriding core.get_timeofday(),
which will conflict.

The second part of the problem is that doing this from a mod requires
constant maintenance of a settings file.

An implementation in core is trivial, however, and solves all of
these problems at virtually no cost: No extra branches in server
steps, and a single branch when minetest.set_time_of_day(), which is
entirely reasonable.

We store the day_count value in env_meta.txt.

The use case is obvious: This change allows mods to create an actual
virtual calendar, or properly account for seasonal changes, etc..

We add a "/days" chatcommand that displays the current day count. No
permissions are needed. It can only retrieve the day count, not
modify it.

Missing `valleys` in `settingtypes.txt` and `minetest.conf.example`.

If the default value of a setting was empty,
it previously generated a trailing space.

This allows mods to perform both asynchronous and synchronous HTTP
requests. Mods are only granted access to HTTP APIs if either mod
security is disabled or if they are whitelisted in any of the
the secure.http_mods and secure.trusted_mods settings.

Adds httpfetch_caller_alloc_secure to generate random, non-predictable
caller IDs so that lua mods cannot spy on each others HTTP queries.

Closes #3748

This makes the functions a bit faster since they don't
have to recreate the tables every invocation, and makes
the code more readable.

Also, document `wallmounted_to_dir`.

The function was implemented but not documented in `lua_api.txt`.

The engine implementation of `print` packs the varargs into a
table and passes the table directly to `table.concat`. If you
pass any value not supported by `table.concat` (particularly
`nil`), the server crashes. This is unexpected behavior, as
`print` is supposed to be able to work with anything.

This patch changes the implementation so it first converts
all arguments using `tostring`, which fixes the issue and
makes the custom `print` function compatible with the stock
Lua behavior.

Replace simple caves with V5 caves, adding unpredictable water and lava
settings and massive caves based on subterrain. Remove fast terrain mode
and accompanying settings. Remove superfluous temperature/humidity
settings. Remove lava/water height setting. Fix errors in humidity
handling and remove humidity_break_point setting. Move cave noises to
generateCaves. Fix minor formatting/naming issues and use
MYMAX/MYMIN/myround.

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.

The save button is now fully functional again when an error message
is shown.

After an invalid value is entered in the settings tab dialog, the GUI
label for the error message that is shown was partly overlapping the
'save' button, so that the top half of the button could not be clicked
on.

Changes:
- Accept setting an empty flags-type value in the settings tab
  if the variable specification permits it
- Don't accept substrings of flag values
  E.g. with values: 'one,two,three', 'hree', 'w', etc. used to
  be accepted. Not any more
- Don't accept flags with random pattern-matching special characters
  E.g. with values: 'one,two,three', 'on.', '(o)[n]e*' etc. used
  to be accepted. Not any more.