container groundwork

Move a couple of instances of container contents manipulation into
their own routines.  Behavior for items disappearing from cursed bags of
holding isn't quite identical but is effectively the same.  I think its
use of stolen_value (or simply the behavior of the latter) is buggy, but
I haven't tried to fix that.  (Cursed bag of holding destroying a player
owned bag containing shopped owned items definitely doesn't work well.)

include/extern.h

@@ -933,6 +933,7 @@ E void FDECL(mimic_hit_msg, (struct monst *, SHORT_P));
 #ifdef GOLDOBJ
 E void FDECL(mkmonmoney, (struct monst *, long));
 #endif
+E void FDECL(bagotricks, (struct obj *));
 
 /* ### mapglyph.c ### */