core: make setting the shutdown watchdog configuration via dbus work

https://bugs.freedesktop.org/show_bug.cgi?id=88284
(cherry picked from commit 81f5fc2d43)

Conflicts:
	src/core/main.c

src/core/main.c

@@ -1839,6 +1839,8 @@ finish:
         pager_close();
 
         if (m) {
+                arg_shutdown_watchdog = m->shutdown_watchdog;
+
                 manager_free(m);
                 m = NULL;
         }