Support SystemD Type=notify

[Deric-W]

By compiling with the optional feature systemd the service representing the push server can be configured with Type=notify.

This allows actions depending on the service being operational (like occ notify_push:setup) to be delayed until things like connections or Unix domain sockets have been established.

The following things might be debatable:

• using an external dependency for notifying SystemD

• notifying SystemD in a blocking manner from within a future

Closes #452

[icewind1991]

If I understand the sd-notify library correctly. If we ignore any error from sending the notification, there shouldn't be any issue with trying to send the notification on non-systemd systems and having the feature enabled by default.

[Deric-W]

When the required environment variable is not present the function just returns Ok(()) which means we don't have to ignore errors.

[github-actions]

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