core: introduce new stop protocol for unit scopes

core: introduce new stop protocol for unit scopes

By specifiy a Controller property when creating the scope a client can
specify a bus name that will be notified with a RequestStop bus signal
when the scope has been asked to shut down, instead of sending SIGTERM
to the scope processes themselves.

https://bugzilla.redhat.com/show_bug.cgi?id=1032695