launchctl load/unload/start/stop are deprecated on macOS since 10.10
and fail silently on modern versions. This replaces them with the
current equivalents:
- load -> bootstrap gui/<uid> <plist>
- unload -> bootout gui/<uid>/<label>
- start -> kickstart gui/<uid>/<label>
- stop -> kill SIGTERM gui/<uid>/<label>
Adds _launchd_domain() helper returning the gui/<uid> target domain.
Updates test assertions to match the new command signatures.
Fixes#4820