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()

Add minetest.register_on_joinplayer and minetest.register_on_leaveplayer,
make adding new callbacks to scriptapi.cpp easier by adding
scriptapi_run_callbacks, also fix a minor bug with PlayerSAO <->
singleplayer mode interaction