Fail queries fast if no DNS servers are configured.

When no DNS servers are configured (and thus there is no chance
that the DNS query will suceed), res_nsend returns early, but
it does not tell the cache that the query has failed.
Therefore, if the caller retries the query, it will block for
PENDING_REQUEST_TIMEOUT (= 20 seconds) waiting for the "existing
query" (which isn't actually doing anything) to complete.

Bug: 18240188
Bug: 18327075
Change-Id: I0df13ff4a17ee65e640be96695a3af31b020963a

libc/dns/resolv/res_send.c

@@ -402,6 +402,10 @@ res_nsend(res_state statp,
 	}
 
 	if (statp->nscount == 0) {
+		// We have no nameservers configured, so there's no point trying.
+		// Tell the cache the query failed, or any retries and anyone else asking the same
+		// question will block for PENDING_REQUEST_TIMEOUT seconds instead of failing fast.
+		_resolv_cache_query_failed(statp->netid, buf, buflen);
 		errno = ESRCH;
 		return (-1);
 	}