Add script to move a backup to another server

i.e. send the ZFS, update db records and copy the config files. The
sql inserts should broadly mirror those done when setting up a new
backup, though with field values matching those in the source database
rather than just using the defaults.

This script has so only been tested for the case of moving a "simple"
backup where a host has a single backup task and no special config.
It's quite likely there'll be bugs to fix for other cases that
we'll find in due course.