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/forward/README.md

@@ -19,6 +19,8 @@ is performed and upstreams will always be considered healthy.
 When *all* upstreams are down it assumes health checking as a mechanism has failed and will try to
 connect to a random upstream (which may or may not work).
 
+This plugin can only be used once per Server Block.
+
 ## Syntax
 
 In its most basic form, a simple forwarder uses this syntax:

plugin/forward/setup.go

@@ -84,7 +84,13 @@ func parseForward(c *caddy.Controller) (*Forward, error) {
 
 	protocols := map[int]int{}
 
+	i := 0
 	for c.Next() {
+		if i > 0 {
+			return nil, plugin.ErrOnce
+		}
+		i++
+
 		if !c.Args(&f.from) {
 			return f, c.ArgErr()
 		}

plugin/forward/setup_test.go

@@ -30,6 +30,8 @@ func TestSetup(t *testing.T) {
 		// negative
 		{"forward . a27.0.0.1", true, "", nil, 0, false, "not an IP"},
 		{"forward . 127.0.0.1 {\nblaatl\n}\n", true, "", nil, 0, false, "unknown property"},
+		{`forward . ::1
+		forward com ::2`, true, "", nil, 0, false, "plugin"},
 	}
 
 	for i, test := range tests {