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

remove svc_send.c file

Module and bundle should only know about services inside its.
Spawn a new process with its own graph when dealing with module/bundle services.

This commit sign the end of the "[WIP] allow to switch between trees" works.
The tree directory was used with s6-rc. Tree are now completely handled by
resolve file. That's means that services are not independent of a tree.
Again earn of flexibility.