Don't reset caches when dns props change.

This happens too frequently.  We should flush a per-interface cache
when it's dns server addrs changes.

Change-Id: I8a691c96ce9a775160ef55ddb8f755d649041583

libc/netbsd/resolv/res_state.c

@@ -170,7 +170,6 @@ RESET_STATE:
         pthread_setspecific( _res_key, NULL );
         return NULL;
     }
-    _resolv_cache_reset(rt->_serial);
     return rt;
 }