Sometimes put rings dropped into sinks in the pipes

aka bury the ring under the sink. Idea from Fredrik Ljungdahl.

src/do.c

@@ -501,6 +501,12 @@ register struct obj *obj;
         pline_The("sink backs up, leaving %s.", doname(obj));
         obj->in_use = FALSE;
         dropx(obj);
+    } else if (!rn2(5)) {
+        freeinv(obj);
+        obj->in_use = FALSE;
+        obj->ox = u.ux;
+        obj->oy = u.uy;
+        add_to_buried(obj);
     } else
         useup(obj);
 }