cryptodisk: Add infrastructure to pass data from cryptomount to cryptodisk modules

cryptodisk: Add infrastructure to pass data from cryptomount to cryptodisk modules

Previously, the cryptomount arguments were passed by global variable and
function call argument, neither of which are ideal. This change passes data
via a grub_cryptomount_args struct, which can be added to over time as
opposed to continually adding arguments to the cryptodisk scan and
recover_key.

As an example, passing a password as a cryptomount argument is implemented.
However, the backends are not implemented, so testing this will return a not
implemented error.

Also, add comments to cryptomount argument parsing to make it more obvious
which argument states are being handled.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>