Catch some error situations when mod used without thinking about it

Most modders would otherwise forget to explicitly define this, and generated nodes aliased from mods would wall-off caves

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)

Nodes in this group will be dropped as items if the node under them or the node in the wallmounted direction is not walkable.