level drain while polymorphed (trunk only)

Noticed when incorporating the "vampire dancing" patch:  losing a level
while polymorphed would subtract from your normal hit points but didn't
affect your monster hit points.  Now they'll lose d8 from max and current,
similar to the amount they increase when gaining a level.

     This also addresses an issue from the newsgroup a few weeks back:
someone mentioned an assumption that Stormbringer drained an amount other
than d8 for monsters who use some other formula for their hit points.  It
wasn't true, but now it will be (approximately).  Most monsters with unusual
hit points aren't subject to level drain, so it shouldn't have much impact.

include/extern.h

@@ -960,6 +960,7 @@ E void FDECL(readmail, (struct obj *));
 
 E boolean FDECL(is_home_elemental, (struct permonst *));
 E struct monst *FDECL(clone_mon, (struct monst *,XCHAR_P,XCHAR_P));
+E int FDECL(monhp_per_lvl, (struct monst *));
 E void FDECL(newmonhp, (struct monst *,int));
 E struct monst *FDECL(makemon, (struct permonst *,int,int,int));
 E boolean FDECL(create_critters, (int,struct permonst *));