New plugin: loop (#1989)

* New plugin: loop

Add a plugin that detects loops. It does this by sending an unique query
to our selves. If we see the query more than twice we stop the process.
If there isn't a loop, the plugin disables it self and becomes a noop
plugin.

Signed-off-by: Miek Gieben <miek@miek.nl>

core/dnsserver/zdirectives.go

@@ -39,6 +39,7 @@ var Directives = []string{
 	"auto",
 	"secondary",
 	"etcd",
+	"loop",
 	"forward",
 	"proxy",
 	"erratic",