[libfko] Don't trundate > 16 byte Rijndael keys

[libfko] Don't trundate > 16 byte Rijndael keys

Significant bug fix to honor the full encryption key length for
user-supplied Rijndael keys > 16 bytes long.  Previous to this bug fix,
only the first 16 bytes of a key were actually used in the encryption/
decryption process even if the supplied key was longer.  The result was
a weakening of expected security for users that had keys > 16 bytes,
although this is probably not too common.  Note that "passphrase" is
perhaps technically a better word for "user-supplied key" in this
context since Rijndael in CBC mode derives a real encryption/decryption
key from the passphrase through a series of applications of md5 against
the passphrase and a random salt.  This issue was reported by Michael T.
Dean.  Closes issue #18 on github.