[DGD] Gurbalib weapon class and general power tuning

[nikoroleva]

I've tinkered with masking do_swing() and query_defense() in combat.c
to achieve some of those things, but never landed on anything I liked.
A few years ago, I added damage hook functions to gurbalib. The
original motivation was to allow for things like "silver weapons do
more damage to vampires", but they could be used for other damage
alteration effects. In any case, swing/defence hooks could be useful.