Add script for moving a backup task rather than a host

For machines like nest-backup and cerebro-backup we have lots of backup
tasks for the same host spread across several zpools, so
move-machine-to-zpool.sh can't be used to migrate the contents of a
failing zpool/disk. This adds a script to move an individual ZFS which is
the target of a backup task to another zpool.

It assumes all necessary parent ZFSes already exist on the target. If
they don't it fails.

It does not yet clean up the old ZFS as it's not had a lot of use.