Avoid multiple dns lookups for the same query

If two or more rapid dns requests for the same server are done
from different threads it turns into separate dns reques, if
the response of the request isn't found in the cache.

This patch avoid multiple request for the same server by
letting subsequents request wait until the first request
has finished.

Change-Id: Ic72ea0e7d3964a4164eddf866feb4357ec4dfe54

libc/private/resolv_cache.h

@@ -95,4 +95,10 @@ _resolv_cache_add( struct resolv_cache*  cache,
                    const void*           answer,
                    int                   answerlen );
 
+/* Notify the cache a request failed */
+extern void
+_resolv_cache_query_failed( struct resolv_cache* cache,
+                   const void* query,
+                   int         querylen);
+
 #endif /* _RESOLV_CACHE_H_ */