Add firmware check and message during SSH login

[m-roberts]

Closes #72

This PR attempts to add a message during each SSH login showing if there is a firmware update available.

Still need to parse the command output without invoking desktop notifications, evaluate performance and produce a human-friendly message about what to do if there is an update.

[m-roberts]

Just thinking about setting this update check as permissable for non-root to work with SSH login... I think that the current sudo requirement is because of the fact that this works with notify-send.

Seems we need to split out the desktop notification management into its own daemon that runs as root so that the check (and update?) script can be invoked as a non-root user.

It seems reasonable to either have a flag to determine backend/use-case, or that the desktop notification daemon is only used if it exists with a stdout fallback.

Also, 'check' is an infinite service, not a one-shot as is needed.