Unregister all shutdown consumers when receiving SIGTERM

Unregister all shutdown consumers when receiving SIGTERM

systemd uses SIGTERM to stop its units. We want to cleanly shut down
the NSC when receiving this signal. Part of that is to unregister all
shutdown consumers. If we don't do this, the NSM will later during its
shutdown phase call all shutdown consumers registered by the NSC. And
if the NSC was no longer running, this would cause the NSM to wait for a
timeout for every single one of these shutdown consumers. Obviously,
that is not desirable.