libblkid: implement zone-aware probing

libblkid: implement zone-aware probing

This patch makes libblkid zone-aware. It can probe the magic located at
some offset from the beginning of some specific zone of a device.

This patch introduces some new fields to struct blkid_idmag. They indicate
the magic location is placed related to a zone and the offset in the zone.

Also, this commit introduces `zone_size` to struct blkid_struct_probe. It
stores the size of zones of a device.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>