Damage System: ability to integrate modifier

[alek13]

As for now, there is nodes with damage of: fire, poison,...
But MineTest does not handle damage_groups for nodes.

We need add ability into Core/damage to handle this.
We use minetest.register_on_player_hpchange(), callback of which can modify changed hp.
So, as a workaround we can just register our own modifers.

In Core/damage

• add ability to pass callback as modifier into on_damage_of()

And then we can use it in our lord_defense to prevent damage of those nodes if we have appropriate defense.