disk/cryptodisk: Add options to cryptomount to support keyfiles

Add the options --key-file, --keyfile-offset, and --keyfile-size to
cryptomount and code to put read the requested key file data and pass
via the cargs struct. Note, key file data is for all intents and purposes
equivalent to a password given to cryptomount. So there is no need to
enable support for key files in the various crypto backends (e.g. LUKS1)
because the key data is passed just as if it were a password.

Signed-off-by: John Lane <john@lane.uk.net>
Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/file.h

@@ -90,6 +90,8 @@ enum grub_file_type
     GRUB_FILE_TYPE_FONT,
     /* File holding encryption key for encrypted ZFS.  */
     GRUB_FILE_TYPE_ZFS_ENCRYPTION_KEY,
+    /* File holding the encryption key. */
+    GRUB_FILE_TYPE_CRYPTODISK_ENCRYPTION_KEY,
     /* File we open n grub-fstest.  */
     GRUB_FILE_TYPE_FSTEST,
     /* File we open n grub-mount.  */