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

Reduce EnvRef:set_node() time tenfold by postponing the dayNightDiff update until it is actually needed

Set default maximum viewing range to a bit more than default server send range to make fog work well