blkid: Add support for LUKS2 and new LABEL attributes.

blkid: Add support for LUKS2 and new LABEL attributes.

This patch adds support for detection of a LUKS2 superblock.

LUKS2 is new version of Linux Unified Key Setup for encrypted
block devices.

LUKS2 contains a binary header and then JSON area for metadata.
Blkid should only parse the binary part, including newly available
optional LABEL and SUBSYSTEM fields.

LABEL is similar to filesystem label. The SUBSYSTEM field is
in principle, just a second label and can be used for specific udev rules
(for example if you have some 3rd party system that activates
volumes automatically, you can mark devices using this attribute).
Both labels are optional.

The magic string and UUID location are intentionally on the same offset
as LUKS v1, so even unpatched blkid now recognizes LUKS2.

Anyway, the code should not parse other versions of the header, so we now
explicitly check for header version and support only version 1 and 2.

Signed-off-by: Milan Broz <gmazyland@gmail.com>