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/hosts/README.md

@@ -11,6 +11,8 @@ file that exists on disk. It checks the file for changes and updates the zones a
 plugin only supports A, AAAA, and PTR records. The hosts plugin can be used with readily
 available hosts files that block access to advertising servers.
 
+This plugin can only be used once per Server Block.
+
 ## Syntax
 
 ~~~

plugin/hosts/setup.go

@@ -69,7 +69,13 @@ func hostsParse(c *caddy.Controller) (Hosts, error) {
 	config := dnsserver.GetConfig(c)
 
 	inline := []string{}
+	i := 0
 	for c.Next() {
+		if i > 0 {
+			return h, plugin.ErrOnce
+		}
+		i++
+
 		args := c.RemainingArgs()
 		if len(args) >= 1 {
 			h.path = args[0]

plugin/hosts/setup_test.go

@@ -57,6 +57,15 @@ func TestHostsParse(t *testing.T) {
 			}`,
 			false, "/etc/hosts", []string{"miek.nl.", "10.in-addr.arpa."}, fall.Root,
 		},
+		{
+			`hosts /etc/hosts {
+				fallthrough
+			}
+			hosts /etc/hosts {
+				fallthrough
+			}`,
+			true, "/etc/hosts", nil, fall.Root,
+		},
 	}
 
 	for i, test := range tests {