salamanders have no legs

However, they could ride.  Marilith is similar, but since it's MZ_LARGE,
that check disallowed riding.  Added a check for slithy(), like
canwearobj() already has.

src/steed.c

@@ -228,7 +228,7 @@ mount_steed(mtmp, force)
 	}
 
 	if (Upolyd && (!humanoid(youmonst.data) || verysmall(youmonst.data) ||
-			bigmonst(youmonst.data))) {
+			bigmonst(youmonst.data) || slithy(youmonst.data))) {
 	    You("won't fit on a saddle.");
 	    return (FALSE);
 	}