Signal systemd when we start shutting down to complement the "READY" notify
that was originally implemented in 97088ebef7
From [sd_notify(3)](https://www.freedesktop.org/software/systemd/man/sd_notify.html#STOPPING=1)
> STOPPING=1
> Tells the service manager that the service is beginning its shutdown. This is useful
> to allow the service manager to track the service's internal state, and present it to
> the user.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
