Add clearing of ZFS labels

ZFS labels were not cleared by GParted when clearing old file system
signatures.

    # wget https://git.kernel.org/cgit/utils/util-linux/util-linux.git/plain/tests/ts/blkid/images-fs/zfs.img.bz2
    # bzip2 -dc zfs.img.bz2 > /dev/sdb1
    [In GParted format to cleared /dev/sdb1]
    # blkid /dev/sdb1
    /dev/sdb1: LABEL="tank" UUID="1782036546311300980" UUID_SUB="13179280127379850514" TYPE="zfs_member"

Update to also zero all 4 ZFS labels.

NOTE:
GParted now writes a little over 1 MiB when clearing old file system
signatures.  As this is performed in the main thread the UI is not able
to respond during this action.  Testing this on a range of USB flash
keys and hard drives found the slowest normal time to write this was
0.25 seconds, with an occasional outlier up to 2.8 seconds from a USB
flash key.  This is considered acceptable.