core: watch PIDs of scope units right after starting them

Scope units don't have a main or control process we can watch, hence
let's explicitly watch the PIDs contained in them early on, just to make
things more robust and have at least something to watch.

src/core/scope.c

@@ -347,6 +347,9 @@ static int scope_start(Unit *u) {
         s->result = SCOPE_SUCCESS;
 
         scope_set_state(s, SCOPE_RUNNING);
+
+        /* Start watching the PIDs currently in the scope */
+        (void) unit_enqueue_rewatch_pids(UNIT(s));
         return 1;
 }