* Implement minetest.register_on_userlimit_check
This new callback permits to bypass the max_users parameter with new mods condition, based on player name or IP
Only one mod needs to permit it.
Move core part for builtin privileges checks to builtin
* Modernize source code: last par
* Use empty when needed
* Use emplace_back instead of push_back when needed
* For range-based loops
* Initializers fixes
* constructors, destructors default
* c++ C stl includes
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)
