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_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,