..and convert such things to human-friendly versions if required.

This is to facilitate extra checks where we want to make numeric
calculations involving quotas and other similar properties.

This will let us use zfs_target as the name of a subtest which
in turn means we would be able to separately log and graph multiple
backup targets associated with a single host.

This change does not affect the current parsing performed when
we input data into postgres: it uses non-anchored regexps to
identify SpaceUsed etc so prepending extra text won't change
anything

Let's not worry if we haven't backed up a machine that has been offline for 3 months (instead of 6 months)

The script runs some SQL which ultimately determines the time at which the last
backup with an exit code of zero was. If there has never been a successful
backup, we ended up reporting a clear dot. Whilst this may lead to a few
transitory red dots immediately after adding a new host, I think this is
preferable to not realising for months that a host isn't being backed up!
See ticket 154527

For offline machines, use the time between last backup and last change of conn dot status to decide on backup status

Forcing arcfour causes us not to be able to back up jessie machines.