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

docs/services/routing.md

@@ -189,6 +189,8 @@ do("POST", "/api/blog/posts/7/comments", true)
 ./bin/acme route:list
 ```
 
+The table is followed by a line saying whether the admin is mounted and under which prefix, for example `admin: mounted at /backend`.
+
 `surf.BuildRouter` returns the same information to Go code through `surf.Router.Routes`:
 
 ```go src=modules/surf/example_test.go#ExampleBuildRouter