middleware/whois: hook it up

Hook it up properly by adding it to the directives list. And add
the Target to the SRV record to actually return valid DNS messages.

core/dnsserver/directives.go

@@ -92,4 +92,5 @@ var directives = []string{
 	"etcd",
 	"kubernetes",
 	"proxy",
+	"whoami",
 }