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.
Yes. `launchctl list` works only for non-startup plists,  and `sudo launchctl list` works only for startup one.