Implement party MP calculation, raise HP cap

Party MP now calculated dynamically based on characters in party
(including benched members); HP cap for party members raised from 999 to
9999.  Currently there's no theoretical cap on enemy HP, although the
kh2Bar gauge will overflow after about 10000 HP or so... might add a
property to the gamedef at some point to allow customizing the boss
lifebar's sector size.