libblkid: udf: add support for APPLICATION_ID

libblkid: udf: add support for APPLICATION_ID

iso9660 superblock code already sets APPLICATION_ID to Application
Identifier from ISO9660 Supplementary Volume Descriptor.

In udf similar meaning has Application Identifier from UDF Primary Volume
Descriptor. Therefore exports it as APPLICATION_ID label.

More tools for generating hybrid ISO+UDF images (e.g. genisoimage, mkisofs
or nero) set Application Identifier in UDF Primary Volume Descriptor to
just empty string. But they set Implementation Identifier in UDF Primary
Volume Descriptor to their tool name.

So if Application Identifier is empty then exports Implementation
Identifier as APPLICATION_ID label.

Please note that Application Identifier and Implementation Identifier are
stored in UDF only in ASCII, not UNICODE.

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