Initial sd_notify support was added in #3963, but that sent signals from
both cmdRun and cmdReload. This approach has two drawbacks:
- Reloads initiated via the API do not send signals.
- The signals are sent from different processes, which requires the
`NotifyAccess=exec` directive in the unit file.
This change moves the NotifyReloading and NotifyReadiness invocations to
Load, which address both of those drawbacks. It also adds a
complimentary NotifyStopping method which is invoked from handleStop.
All the notify methods are defined in a notify package to avoid an