* 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
This privilege allows map protection bypassing for server operators
and world moderators.
Initially I had thought that bypassing protection mods would have been
something that could entirely be done inside mods and minetest_game,
but the concept of protection is defined in core, in the code of
core.is_protected().
I don't feel that it would be logical to introduce a protection
concept in core, but not some way around that for server operators
to maintain map parts that need fixing, de-griefing or cleanup.
Others had noticed the same problems, and proposed a patch to
minetest_game. That patch is fine by itself, but it fails to add
protection bypass functionality for digging normal nodes and placing
nodes.
So, instead, we indroduce the new priv "protection_bypass" in core,
and modify 'on_place_node' and 'node_dig' to allow bypassing node
protections if the player holds this priv.
This priv was tested with protector redo by tenplus1.
A followup patch to Minetest Game will include allowing special checks
for doors, trapdoors, chests in Minetest Game.
Protection mods will likely want to mimic the changes in their relevant
code sections.
