Adding support for explicit no-cache policy for positive/negative dns…

… queries

plugin/cache/cache.go

@@ -203,12 +203,16 @@ func (w *ResponseWriter) set(m *dns.Msg, key uint64, mt response.Type, duration
 	// and key is valid
 	switch mt {
 	case response.NoError, response.Delegation:
-		i := newItem(m, w.now(), duration)
-		w.pcache.Add(key, i)
+		if w.pcap > 0 {
+			i := newItem(m, w.now(), duration)
+			w.pcache.Add(key, i)
+		}
 
 	case response.NameError, response.NoData:
-		i := newItem(m, w.now(), duration)
-		w.ncache.Add(key, i)
+		if w.ncap > 0 {
+			i := newItem(m, w.now(), duration)
+			w.ncache.Add(key, i)
+		}
 
 	case response.OtherError:
 		// don't cache these