discover: Recognise and open LUKS encrypted partitions

discover: Recognise and open LUKS encrypted partitions

Handle devices encrypted with LUKS and call cryptsetup to open them if a
client sends the associated password.
If a new device has the "crypto_LUKS" filesystem type it is marked as a
LUKS device and sent to clients but further discovery is not performed.
Once a client sends the device's password cryptsetup is called to open
it. The opened device will appear separately, so the source device is
"forgotten" at this point and then the newly opened device is treated as
a normal partition. On destruction the device is "closed" with
cryptsetup so that discovery can start from the beginning.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>