handling zroot/var/empty readonly=on

[dch]

zfs dataset /var/empty was a valid dataset during 10.x FreeBSD, so we
can expect it to turn up in the wild. This could be handled as follows:

• if /var/empty/.zfs/snapshot dir exists then:

# zfs snapshot zroot/var/empty@${some_pkgbasify_id}
# zfs set readonly=off zroot/var/empty

.. do pkgbasify magic
.. now clean up

# zfs rollback -f zroot/var/empty@${some_pkgbasify_id}
# zfs set readonly=on zroot/var/empty