This reverts commit c09d026f.

Add find_surface_nodes_in_area LUA call which permit to only get the nodes which touch air. This permit to massively improve performance for mods like plantlife

Directely or indirectly optimises the following functions:

* MapBlockMesh::MapBlockMesh
* MapBlockMesh::getTileInfo
* MapBlockMesh::makeFastFace
* MapBlockMesh::getSmoothLightCombined

wrapDegrees() (renamed to modulo360f)
wrapDegrees_0_360
wrapDegrees_180

Minor errors were present in previous versions; see issue #2328

Rebased by Zeno- (conflict in lua_api.txt)

Linux distributions prefer to link against a shared version of the Irrlicht
engine instead of using embedded code copies of the same. Search for this
shared version first and use that but fall back to the static version if it
does not exist.

This also fixes https://github.com/minetest/minetest/issues/2163

ok @Zeno- @nerzhul

* Remove --enable-unittests and --disable-unittests and add --do-unittests function
* --do-unittests function will exit 0 on success.
* minetest and minetestserver binaries are launched with --do-unittests in travis build.

Add large pseudorandom caves below -256
De-link terrain level from water_level
Cavegen: Mgv5 large cave code

* prefix apt-get with sudo
* git clone minetest_game without going to folder
* execute minetest binary without going into bin folder

InventoryAction is in inventorymanager.h

Fixes #2037

Client now informs about incompatible servers from the list, this permits to prevent the protocol movements.
Server announces its supported protocol versions to master server

Fix unused (and so, broken) enable_rollback_recording. This option must be reloaded at server loop but loaded when server starts, for data consistency (not a hot load variable)
ok @ShadowNinja

Change many useless std::list into Environment class (m_players), ABMHandler and ActiveBlockList::update
This improve looping performances.