Commit edea370222 (portable: remove drop-in configs even if the main unit file does not exist)
taught detach to handle leftover .service.d directories
after the main unit symlink was removed.
portable_get_state_internal() still had the same blind
spot: it only considered regular unit entries, so portablectl
is-attached could report detached while a portable drop-in directory was still present.
Handle those drop-in-only entries like detach does, preserve
the existing unit-file based enabled checks when the
main unit file is still present, and add TEST-29 coverage.
Signed-off-by: dongshengyuan <dongshengyuan@uniontech.com>