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/README.md

@@ -16,6 +16,8 @@ to deploy CoreDNS in Kubernetes](https://github.com/coredns/deployment/tree/mast
 [stubDomains and upstreamNameservers](http://blog.kubernetes.io/2017/04/configuring-private-dns-zones-upstream-nameservers-kubernetes.html)
 are implemented via the *proxy* plugin and kubernetes *upstream*. See example below.
 
+This plugin can only be used once per Server Block.
+
 ## Syntax
 
 ~~~

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

plugin/kubernetes/setup_test.go

@@ -388,7 +388,7 @@ func TestKubernetesParse(t *testing.T) {
 			`kubernetes coredns.local
 kubernetes cluster.local`,
 			true,
-			"only one kubernetes section allowed per server block",
+			"this plugin",
 			-1,
 			0,
 			defaultResyncPeriod,