libblkid: iso9660: add support for VOLUME_SET_ID and DATA_PREPARER_ID

libblkid: iso9660: add support for VOLUME_SET_ID and DATA_PREPARER_ID

udf superblock code already sets VOLUME_SET_ID to Volume Set Identifier
from UDF Primary Volume Descriptor.

In iso9660 similar meaning has Volume Set Identifier in ISO9660
Primary (or Supplementary Joliet) Volume Descriptor. Therefore exports it
as VOLUME_SET_ID label.

In ISO9660 Primary (and Supplementary Joliet) Volume Descriptor exists
another member Data Preparer Identifier which is currently not parsed by
iso9660 superblock code.

To have iso9660 superblock code feature complete export also Data Preparer
Identifier as DATA_PREPARER_ID label. UDF does not have equivalent of Data
Preparer Identifier.

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