Add ZFS detection to libblkid

Add ZFS detection to libblkid

This patch adds ZFS filesystem detection to libblkid.

It probes for VDEV_BOOT_MAGIC in the first 2 ZFS labels in big-endian
and little-endian formats.
Unfortunately the probe table doesn't support probing from the end of
the device, otherwise we could also probe in the 3rd and 4th labels (in
case the first 2 labels were accidentally overwritten)..

Eventually we would set the UUID from the ZFS pool GUID and the LABEL tag
from the pool name, but that requires parsing an XDR encoding of the pool
configuration which is not trivial.

Signed-off-by: Ricardo M. Correia <Ricardo.M.Correia@Sun.COM>
Signed-off-by: Andreas Dilger <adilger@sun.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>