The backup service can now be snapshotted and shut down cleanly

The backup server was the only machine on the platform without the small helper the host uses to talk to it. Three things followed from that: its own snapshots could only ever be crash-consistent, there was no way to shut it down tidily when power is lost, and there was no way in if normal access ever failed.

The helper is installed and the host-side connection wired up, during a three-minute restart taken in a window with no backup, verification or housekeeping running. Snapshots of the backup server can now be taken with its filesystems briefly quiesced, so they are consistent rather than merely recent.