Makes the result compatible with the minetest.get_inventory(location) param.

On the lua side, notably minetest.env:<function>(<args>) should now
be replaced by minetest.<function>(<args>).
The old way is and will stay supported for a long time.

Also:
Update and clean up lua_api.txt (by celeron55)
Move EnvRef to lua and remove add_rat and add_firefly (by kahrl)
Add separate src/util/CMakeLists.txt, other minor fixes (by kahrl)

Make raillike nodes connect to any other raillike nodes if both are in the group connect_to_raillike

Concatenate the thread id as string instead of adding it to the pointer

Mods are placed in $path_<user/share>/mods
They can be enabled per world in world.mt or the configure world window