* grub-core/fs/udf.c: Add support for UUID

* grub-core/fs/udf.c: Add support for UUID

Use same algorithm as in libblkid from util-linux v2.30.

1. Take first 16 bytes from UTF-8 encoded string of VolumeSetIdentifier
2. If all bytes are hexadecimal digits, convert to lowercase and use as UUID
3. If first 8 bytes are not all hexadecimal digits, convert those 8 bytes
   to their hexadecimal representation, resulting in 16 bytes for UUID
4. Otherwise, compose UUID from two parts:
   1. part: converted first 8 bytes (which are hexadecimal digits) to lowercase
   2. part: encoded following 4 bytes to their hexadecimal representation (16 bytes)

So UUID would always have 16 hexadecimal digits in lowercase variant.

According to UDF specification, first 16 Unicode characters of
VolumeSetIdentifier should be unique value and first 8 should be
hexadecimal characters.

In most cases all 16 characters are hexadecimal, but e.g. MS Windows
format.exe set only first 8 as hexadecimal and remaining as fixed
(non-unique) which violates specification.