tree-wide: make sure to flush/close all bus connections when our daemons exit

Let's make sure everything is written out ebfore we exit, so that no
messages keep our bus connections referenced.

Fixes: #11462

src/machine/machined.c

@@ -81,7 +81,7 @@ static Manager* manager_unref(Manager *m) {
 
         bus_verify_polkit_async_registry_free(m->polkit_registry);
 
-        sd_bus_unref(m->bus);
+        sd_bus_flush_close_unref(m->bus);
         sd_event_unref(m->event);
 
         return mfree(m);