libblkid: add blkid_probe_{set,get}_hint()

libblkid: add blkid_probe_{set,get}_hint()

Add new API for additional probing hints (offsets). The idea is that
probing function can optionally use the hint to detect superblock or
partition table if on unusual place where the library does not expects (etc.)

The hints use strings as identifiers (e.g. "last-medium-session") to
make it extendable and prober specific (if necessary). The function
blkid_probe_set_hint() always add a new hint, so you can specify more
offsets with the same (hint) name.

The function blkid_probe_get_hint() is not public. For now it returns
the first hint only.

Addresses: https://github.com/karelzak/util-linux/issues/1161
Signed-off-by: Karel Zak <kzak@redhat.com>