plugin/deprecated: add an easy way to error on deprecated plugin (#1575)

* plugin/deprecated: add an easy way to error on depcated plugin

* lint

* better docs

* slightly more docs

plugin/deprecated/setup.go

+// Package deprecated is used when we deprecated plugin. In plugin.cfg just go from
+//
+// startup:github.com/mholt/caddy/startupshutdown
+//
+// To:
+//
+// startup:deprecated
+//
+// And things should work as expected. This means starting CoreDNS will fail with an error. We can only
+// point to the release notes to details what next steps a user should take. I.e. there is no way to add this
+// to the error generated.
+package deprecated
+
+import (
+	"errors"
+
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+// removed has the names of the plugins that need to error on startup.
+var removed = []string{"startup", "shutdown"}
+
+func setup(c *caddy.Controller) error {
+	c.Next()
+	x := c.Val()
+	return plugin.Error(x, errors.New("this plugin has been deprecated"))
+}
+
+func init() {
+	for _, plugin := range removed {
+		caddy.RegisterPlugin(plugin, caddy.Plugin{
+			ServerType: "dns",
+			Action:     setup,
+		})
+	}
+}