Stop blisters from lowering max HP

Blisters currently cause damage to max HP. However, based on a [forum post from Shoes](http://www.cataclysmdda.com/smf/index.php?topic=1014.msg10970#msg10970), blisters were never supposed to cause permanent harm. Likewise, we really should not be lowering max HP without alerting the player.

The easiest resolution seems to be turning that into current HP damage, which is what I've done here. This should be revisited when we eventually have a more interesting wound system.