Add --backup and --restore operations

[ValShaped]

Squish the Rwfus basedir (default /home/.rwfus) into a tarball, and unsquish a tarball onto the Rwfus basedir

Syntax:

rwfus --restore /path/to/backup.tar
rwfus --backup /path/to/backup.tar

[ValShaped]

With rwfus now storing all the overlay stuff in sparse disk images, this should be almost trivial now. Hopefully will have this done in the next day or so.