feat(surf): print the serve URL and admin status on startup

- Add surf.Router.AdminPrefix (additive): the admin mount path and true
  when BuildRouter mounted the cabana admin, false otherwise
- serve binds with net.Listen before printing, so a busy port fails
  without a misleading URL, then prints the listen address with a
  browser URL and an admin status line: the admin URL, a warning naming
  summer admin:create when backend_users is empty or missing, or
  admin: off with the steps to add an admin controller
- The administrator count is bounded by a 2 s timeout and never fails boot
- route:list ends with the same admin status line
- surf and cabana READMEs and the console and routing docs describe the output

modules/surf/router.go

@@ -60,6 +60,8 @@ type Router struct {
 	uploadBytes  int64
 	built        map[string]pact.Middleware
 	app          *backpack.App
+	adminPrefix  string
+	adminMounted bool
 }
 
 var (
@@ -556,6 +558,8 @@ func BuildRouter(app *backpack.App, plugins []party.Plugin) (*Router, error) {
 		if err := r.checkAdminPrefix(admin.Prefix); err != nil {
 			return nil, err
 		}
+		r.adminPrefix = admin.Prefix
+		r.adminMounted = true
 	}
 	for _, rt := range r.routes {
 		if _, err := r.wrap(rt); err != nil {