ui/ncurses: Add prompt for LUKS device password

Implement device_add() in cui_client_ops and use this interface to
recognise when the server notifies the client of an encrypted device. A
"device header" will be created for this device and added to the menu
with no associated boot options.

The nc-auth prompt is extended to ask for a disk password when the device
header for an encrypted device is selected.
Assuming the password is correct pb-discover will remove the original
device and notify the client about the newly opened device, which will
be reflected in the menu.

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