fix #K4060 - "you walk quietly" while riding

Donning elven boots while riding and not already stealthy, you'd get
the message "you walk quietly" when not walking at all.  Instead of
just changing the message, make riding a non-flying steed block
stealth.  Riding a flying steed (or one you take aloft with an amulet
of flying) does not.  It would have been quite a bit simpler to have
made riding anything block stealth, but the hard part is done.

src/mon.c

@@ -5054,6 +5054,8 @@ newcham(
             }
         }
     }
+    if (mtmp == u.usteed)
+        poly_steed(mtmp, olddata);
 
     return 1;
 }