Mgv6: Enable snowbiomes by default. Double biome noise spread. 3 octaves, 0.5 persistence for humidity

Use v arrows except where there is only one line between two boxes,
and other improvements
(Yea could use ↓ but its an ASCII art not an Unicode art).

Mgv5/v7: Fix generateBiomes biome recalculation logic Biomegen down to y = -192 for mgv5 deep oceans. Improve code

Also fix a startup error caused by s_security.cpp

Thanks to @UltimateNate for pointing this out :)

This allows newer versions to add additional information that isn't
read by us, but still in a backwards compatible manner.

Also, clean up surrounding code style
Replace by-value parameter passing with const refs when possible
Fix post-increment of iterators

libstdc++ doesn't include <cstring> inside of <string>, unlike libc++.

Its a possible mistake to log in to a server with twice the same name.
Before, it triggered a server wide error message, now it logs to actionstream.

Work around irrlicht bug and hide nametag if its alpha is set to 0.
Thanks @TeTpaAka for pointing out workaround.

Specifically, the added rule matches .kate-swp files.

Commit 11a96e49
commented out version parsing, resulting in the version byte being parsed as part of the color,
making nametags dark grey.
Re-introduces version parsing and ignores the packet if the version is more recent than ours,
fixing the unused variable warning (should have been done anyway).

This improves performance of MapBlock::raiseModified by a factor of 6.
Also, clean up mapblock.h a bit and inline small functions.

This adds the ability to grab 'slices' of noise calculated by PerlinNoiseMap.
Retrieving smaller slices of noise from the computation result as needed
optimizes memory usage while maintaining a reasonable amount of CPU overhead.

Due to compatibility concerns, this is temporarily disabled.

  * The time command now accepts parameters in the form <hour>:<minute>,
    and if invoked with no parameters returns the current time in said format.

Also rename method to better match what it does.

This enables srp.

Before this commit autofast for Android was ignored if the user was sneaking, digging or building. There is no reason for this and it impedes movement control

Android terminated immediately after start since commit
181f7baa
because library wasn't loaded, and libminetest.so required
libgmp.so.10 instead of libgmp.so.

The second issue has been solved by the linker flags.