hiding under a cockatrice corpse

Reported a while back, a (stonable) hiding monster will hide at a location
containing only a cockatrice corpse.  While it would be interesting to
allow monsters to try, and stone themselves as a result, I chose the
simpler fix which is to not have monsters hide in such situations.  I found
the hiding code was duplicated in several places, so I moved it into a new
hideunder() function that works for both the hero and monsters.

src/monmove.c

@@ -1181,9 +1181,7 @@ postmov:
 		   usually set mundetected unless monster can't move.  */
 		if (mtmp->mundetected ||
 			(mtmp->mcanmove && !mtmp->msleeping && rn2(5)))
-		    mtmp->mundetected = (ptr->mlet != S_EEL) ?
-			OBJ_AT(mtmp->mx, mtmp->my) :
-			(is_pool(mtmp->mx, mtmp->my) && !Is_waterlevel(&u.uz));
+		    (void) hideunder(mtmp);
 		newsym(mtmp->mx, mtmp->my);
 	    }
 	    if (mtmp->isshk) {