libblkid: Add detection of FileVault2 partitions

libblkid: Add detection of FileVault2 partitions

FileVault is full disk encryption format based on Apple
Core Storage, used in older macOS versions.

This patch allows automatic detection of Core Storage partition
to be later used with existing cryptsetup support for FileVault2.

Note that we intentionally ignore generic Core Storage; only
specific configuration is detected as cs_fvault2 type here.

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