Do not warn the user about this feature. It only used on very specific case like module migration process.
Badly used, this features can easily broke the sanity of the system state

s6-supervise do not deal with oneshot service. If we send a -r signal, oneshot service are never brought up again

add fdholder and oneshotd service at start/stop of scandir. Be able to be session leader at start time for regular user