disk/diskfilter: Introduce the "cryptocheck" command

disk/diskfilter: Introduce the "cryptocheck" command

This command examines a given diskfilter device, e.g., an LVM disk,
and checks if underlying disks, physical volumes, are cryptodisks,
e.g., LUKS disks, this layout is called "LVM-on-LUKS".

The return value is 0 when all underlying disks (of a given device)
are cryptodisks (1 if at least one disk is unencrypted or in an
unknown state).

Users are encouraged to include the relevant check before loading
anything from an LVM disk that is supposed to be encrypted.

This further supports the CLI authentication, blocking bypass
attempts when booting from an encrypted LVM disk.

Signed-off-by: Maxim Suhanov <dfirblog@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>