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

src/core/scope.h

@@ -55,6 +55,8 @@ struct Scope {
 
         usec_t timeout_stop_usec;
 
+        char *controller;
+
         Set *pids;
 
         sd_event_source *timer_event_source;