plugin/tsig: new plugin TSIG (#4957)

* expose tsig secrets via dnsserver.Config
* add tsig plugin

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/transfer/setup.go

@@ -28,8 +28,10 @@ func setup(c *caddy.Controller) error {
 	})
 
 	c.OnStartup(func() error {
+		config := dnsserver.GetConfig(c)
+		t.tsigSecret = config.TsigSecret
 		// find all plugins that implement Transferer and add them to Transferers
-		plugins := dnsserver.GetConfig(c).Handlers()
+		plugins := config.Handlers()
 		for _, pl := range plugins {
 			tr, ok := pl.(Transferer)
 			if !ok {