buglist: cutting Shopkeeper the long worm

Cutting a shopkeeper poly'd as a long worm would generate strange messages
and could result in a crash.  cutworm didn't deal with all the intricacies
of duplicating a monster.   Fixed by changing cutworm() to use clone_mon()
to do most of its dirty work.  It seems to me that without this change,
cutting a tame long worm could also have similar bad effects.
Other side effects of this change:
- clone_mon now takes x,y coordinates, 0,0 results in previous behavior
- clone_mon no longer always makes cloned monsters tame/peaceful if player
  caused the clone, using the same formula previously in cutworm.  Someone
  else may wish to tweak this for gremlins.
- clone_mon will christen the new mon with the old shopkeeper's name, even
  though clones are never shopkeepers (game can't handle 2 for a shop)
- cutworm can now be called during conflict or pet combat, although I
  added no such calls (yet)

src/potion.c

@@ -2008,7 +2008,7 @@ struct monst *mon,	/* monster being split */
 		You("multiply%s!", reason);
 	    }
 	} else {
-	    mtmp2 = clone_mon(mon);
+	    mtmp2 = clone_mon(mon, 0, 0);
 	    if (mtmp2) {
 		mtmp2->mhpmax = mon->mhpmax / 2;
 		mon->mhpmax -= mtmp2->mhpmax;