Add after_destruct and cache the existence of on_construct, on_destruct and after_destruct for quick skipping when a node does not have them

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

Reimplement authentication handler in Lua; now we have 1) infinite privilege names, 2) minetest.register_authentication_handler()