Thrust Resistance lowered for weaker Skeletons

As a first step toward making Short Sword Fighting less painful for the
HP range in which players usually need to work it, thrust resistance has
been reduced to zero for battered skeletons, 20 for skeletons, and 40
for tusked skeletons.

These numbers are intended to be augmented by all skeletons' 20%
weakness to holy, so that holy enchanted short swords can reach
effective levels against these mid level monsters.