ZFS label detection improvements.

In order to not let zfs code mistakenly analyze a block device as a ZFS
filesystem when it contains some other filesystem,let's analyze better
the label and fail as soon as possible.  In details:

     * Check the magic number of the zio_eck_t in the vdev_phys_t structure,
       which is the first possible magic number met when analyzing a block
       device.
     * Verify the SHA256 checksum of the vdev_phys_t structure, which
       contains the nvlists that are going to be read later.

Signed-off-by: Massimo Maggi<me@massimo-maggi.eu>