Instead of collecting all objects within a huge radius (which could be
a big value), just register each player that connects and give back the
current hold list.

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)

Add ObjRef:is_player() and modify ObjRef:get_player_name() to always return a string to aid better inter-object compatibility of code that assumes objects to be players