Use sd_notify method to sync listener with supervisory process
This patch performs the equivalent of sd_notify(3)
once the listener is operational so that other processes can synchronise with the server. This does nothing if NOTIFY_SOCKET
is not defined in the environment.
The use case I have in mind is when calypso is used in tests but this could also be useful for systemd users, I imagine.
Sorry I don't know anything about Python packaging so that side of things might need fixing up!
Closes: #1093029