sd-bus: introduce new sd_bus_default_flush_close() call

If code enqueues a message on one of the default busses, but doesn't
sync on it, and immediately drops the reference to the bus again, it
will stay queued and consume memory. Intrdouce a new call
sd_bus_default_flush_close() that can be invoked at the end of programs
(or threads) and flushes out all unsent messages on any of the default
busses.

src/systemctl/systemctl.c

@@ -7705,5 +7705,7 @@ finish:
         strv_free(arg_states);
         strv_free(arg_properties);
 
+        sd_bus_default_flush_close();
+
         return r < 0 ? EXIT_FAILURE : r;
 }