Implement customizable HP gauge sector size

New bossHPPerBar and partyHPPerBar members in the gamedef.  This will
make it easier in the future if I decide to change the HP scale.
Centralize damage calculation

All damage calculations are now performed by a single function,
including recoil damage.  Since the basic structure of the damage
formula is always the same regardless of damage type, this makes
maintenance (and experimentation!) much easier.  Why I didn't think to
do this a long time ago I don't know...