( int hash, string password, string salt, int bytes )
mhash_keygen_s2k() generates a key that is
bytes long, from a user given password.
This is the Salted S2K algorithm as specified in the OpenPGP
document (RFC 2440). That algorithm will use the specified
hash algorithm to create the key.
The salt must be different and random
enough for every key you generate in order to create different keys.
That salt must be known when you check the keys, thus it is
a good idea to append the key to it. Salt has a fixed length
of 8 bytes and will be padded with zeros if you supply less bytes.
Keep in mind that user supplied passwords are not really suitable
to be used as keys in cryptographic algorithms, since users normally
choose keys they can write on keyboard. These passwords use
only 6 to 7 bits per character (or less). It is highly recommended
to use some kind of transformation (like this function) to the user