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/errors/README.md

@@ -8,6 +8,8 @@
 
 Any errors encountered during the query processing will be printed to standard output.
 
+This plugin can only be used once per Server Block.
+
 ## Syntax
 
 ~~~

plugin/errors/setup.go

@@ -37,7 +37,13 @@ func setup(c *caddy.Controller) error {
 func errorsParse(c *caddy.Controller) (errorHandler, error) {
 	handler := errorHandler{}
 
+	i := 0
 	for c.Next() {
+		if i > 0 {
+			return handler, plugin.ErrOnce
+		}
+		i++
+
 		args := c.RemainingArgs()
 		switch len(args) {
 		case 0:

plugin/errors/setup_test.go

@@ -12,21 +12,14 @@ func TestErrorsParse(t *testing.T) {
 		shouldErr            bool
 		expectedErrorHandler errorHandler
 	}{
-		{`errors`, false, errorHandler{
-			LogFile: "stdout",
-		}},
-		{`errors stdout`, false, errorHandler{
-			LogFile: "stdout",
-		}},
-		{`errors errors.txt`, true, errorHandler{
-			LogFile: "",
-		}},
-		{`errors visible`, true, errorHandler{
-			LogFile: "",
-		}},
-		{`errors { log visible }`, true, errorHandler{
-			LogFile: "stdout",
-		}},
+		{`errors`, false, errorHandler{LogFile: "stdout"}},
+		{`errors stdout`, false, errorHandler{LogFile: "stdout"}},
+		{`errors errors.txt`, true, errorHandler{LogFile: ""}},
+		{`errors visible`, true, errorHandler{LogFile: ""}},
+		{`errors { log visible }`, true, errorHandler{LogFile: "stdout"}},
+		{`errors
+		errors `, true, errorHandler{LogFile: "stdout"}},
+		{`errors a b`, true, errorHandler{LogFile: ""}},
 	}
 	for i, test := range tests {
 		c := caddy.NewTestController("dns", test.inputErrorsRules)