Improve plist_caveats and brew-services

1. Use `brew services` to replace `launchctl` in plist_caveats.
2. Minor bug fix for brew-services.

Note there's still a bug in brew-services, see https://github.com/Homebrew/homebrew/issues/30628.
The fix is just removing it as it requires `sudo`. If it's OK, I will create a new PR.

Also maybe I will create a separate PR for brew-services bug fix.