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/dnssec/setup_test.go

@@ -61,6 +61,8 @@ func TestSetupDnssec(t *testing.T) {
 				key file
 			}`, true, []string{"example.org."}, nil, defaultCap, "argument count",
 		},
+		{`dnssec
+		  dnssec`, true, nil, nil, defaultCap, ""},
 	}
 
 	for i, test := range tests {