simplify interface of service_is_g and its functionality, provide service_get_treename as replacement

This option come from previous behavior. It not the role of enable process
to deal with parse option.
Also, this change allow user to quickly switch between trees without the need
to force a parse process.
The necessary field at resolve file of the service is now handled by enable
process.

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