libblkid: allow to specify offset defined by hint for blkid_probe_get_idmag()

libblkid: allow to specify offset defined by hint for blkid_probe_get_idmag()

Extends struct blkid_idmag with a new member hoff which specify name of the
hint used for blkid_probe_get_hint() function to retrieve offset to kboff.

blkid_probe_get_idmag() will use supplied hint offset in bytes as a start
position from which the final offsets for magic strings are calculated.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>