return an error for multiple use of some plugins (#1559)

* plugins: Return error for multiple use of some

Return plugin.ErrOnce when a plugin that doesn't support it, is called
mutliple times.

This now adds it for: cache, dnssec, errors, forward, hosts, nsid.
And changes it slightly in kubernetes, pprof, reload, root.

* more tests

plugin/kubernetes/setup.go

@@ -71,12 +71,18 @@ func (k *Kubernetes) RegisterKubeCache(c *caddy.Controller) {
 }
 
 func kubernetesParse(c *caddy.Controller) (*Kubernetes, error) {
-	var k8s *Kubernetes
-	var err error
-	for i := 1; c.Next(); i++ {
-		if i > 1 {
-			return nil, fmt.Errorf("only one kubernetes section allowed per server block")
+	var (
+		k8s *Kubernetes
+		err error
+	)
+
+	i := 0
+	for c.Next() {
+		if i > 0 {
+			return nil, plugin.ErrOnce
 		}
+		i++
+
 		k8s, err = ParseStanza(c)
 		if err != nil {
 			return k8s, err