Use dns.port argument as default

Make DefaultPort just be 53. And use the value of Port to make the zone
stanzas complete (instead of defaulting to 53 always). This allows you
to override the port with dns.port.

core/dnsserver/address.go

@@ -39,7 +39,7 @@ func normalizeZone(str string) (zoneAddr, error) {
 	}
 
 	if port == "" {
-		port = "53"
+		port = Port
 	}
 
 	return zoneAddr{Zone: strings.ToLower(dns.Fqdn(host)), Port: port}, err