libblkid: zfs: Use nvlist for detection instead of Uber blocks

libblkid: zfs: Use nvlist for detection instead of Uber blocks

Currently, blkid relies on the presence of Uber blocks to detect ZFS
partition types. However, Uber blocks are not consistently dumped for
cache and spare vdevs, particularly in pools created prior to
https://github.com/openzfs/zfs/commit/d9885b3. Additionally, indirect
vdevs are incorrectly detected by blkid due to the presence of Uber
blocks in the label. ZFS itself does not depend on Uber blocks either
when reading ZFS labels; instead, it parses the nvlist.
This commit aligns blkid's approach with ZFS by parsing the nvlist in
the label to detect ZFS partition types, requiring at least one valid
label for successful detection. This change also ensures compatibility
with wipefs, as it now uses nvlist headers for offsets instead of the
Uber Magic offset. Consequently, running wipefs -a will zero out the
nvlist header in each label, fully removing the ZFS partition type and
making the pool unimportable. Previously, wipefs -a  did not clear all
the Uber blocks or delete all nvlist headers, allowing pools to remain
importable even after wiping.

Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>