Audit rloc()

Most of the time, rloc() is used for teleporting monsters and it's not a
big deal if they can't find somewhere to go. In a few cases, it is. I
went through all the callsites and made calls to rloc() not cause
impossible()s if they don't need to.

Fixes a bug/suite of bugs reported by ais523.

src/dig.c

@@ -63,7 +63,7 @@ boolean waslit, rockit;
             return;                   /* don't cover the portal */
         if ((mtmp = m_at(x, y)) != 0) /* make sure crucial monsters survive */
             if (!passes_walls(mtmp->data))
-                (void) rloc(mtmp, FALSE);
+                (void) rloc(mtmp, TRUE);
     } else if (lev->typ == ROOM)
         return;