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/pprof/README.md

@@ -16,6 +16,8 @@ For more information, please see [Go's pprof
 documentation](https://golang.org/pkg/net/http/pprof/) and read
 [Profiling Go Programs](https://blog.golang.org/profiling-go-programs).
 
+This plugin can only be used once per Server Block.
+
 ## Syntax
 
 ~~~

plugin/pprof/setup.go

@@ -19,12 +19,15 @@ func init() {
 }
 
 func setup(c *caddy.Controller) error {
-	found := false
 	h := &handler{addr: defaultAddr}
+
+	i := 0
 	for c.Next() {
-		if found {
-			return plugin.Error("pprof", c.Err("pprof can only be specified once"))
+		if i > 0 {
+			return plugin.Error("pprof", plugin.ErrOnce)
 		}
+		i++
+
 		args := c.RemainingArgs()
 		if len(args) == 1 {
 			h.addr = args[0]
@@ -39,7 +42,6 @@ func setup(c *caddy.Controller) error {
 		if c.NextBlock() {
 			return plugin.Error("pprof", c.ArgErr())
 		}
-		found = true
 	}
 
 	pprofOnce.Do(func() {