switch-to-configuration-ng: fix systemd reexec/reload
According to systemd's D-Bus API documentation, a `Reloading` signal
will be sent with the value `false` will be sent when reloading has
finished (indicating that reloading is not active). The entire
reloading sequence has two signals, the with the first value
indicating that reloading has started (active = true). Thus, we must
wait for the active value to be false before we can continue.
(cherry picked from commit de220b0a65d67140ad599cc41e92a79687e81c5a)
![github-actions[bot]](https://avatars.githubusercontent.com/in/15368?v=4&s=60)