New APIs: Support for creating LUKS and managing keys.

New APIs: Support for creating LUKS and managing keys.

This commit adds four APIs for creating new LUKS devices
and key management.  These are:

  luks_format         Format a LUKS device with the default cipher.
  luks_format_cipher  Format with a chosen cipher.
  luks_add_key        Add another key to an existing device.
  luks_kill_slot      Delete a key from an existing device.

This enables all the significant functionality of the
cryptsetup luks* commands.

Note that you can obtain the UUID of a LUKS device already
by using vfs-uuid.

This also includes a regression test covering all the LUKS
functions.