Add two new variables:
file_key_management_digest={sha1|sha224|sha256|sha384|sha512}
This specify the digest function to use in key derivation of the key
used for decryption of the keyfile.
file_key_management_use_pbkdf2=N
This specify whether pbkdf2 is used in the key derivation, and if
so (N != 0), how many iterations.
