R919 - applying lances

Fixes 2 bugs:
1) an impossible() could occur if you applied a lance against a long worm
because the code uses thitmonst to do the hitting, but didn't set bhitpos,
which is required before calling thitmonst.
Add the missing assignment.
2) applying a lance would never mark a knight as a caitiff.  Added a new
check_caitiff function and called it from the 2 existing checks and in
the lance code.

include/extern.h

@@ -1983,6 +1983,7 @@ E void NDECL(u_init);
 
 E void FDECL(hurtmarmor,(struct monst *,int));
 E boolean FDECL(attack_checks, (struct monst *,struct obj *));
+E void FDECL(check_caitiff, (struct monst *));
 E schar FDECL(find_roll_to_hit, (struct monst *));
 E boolean FDECL(attack, (struct monst *));
 E boolean FDECL(hmon, (struct monst *,struct obj *,int));