Allow encrypting a decrypted secret key, which is especially useful for
generating a new encrypted secret key. Defaults to AES256, S2K
iter+salt SHA512, always uses s2k_useage 254 with sha1 integrity
protection of the encrypted key material.
Also add an example to parallel keygen.php that generates a key and then
encrypts it with a passphrase.
While this message may be more helpful, it will break some cases, such
as when there are multiple ciphers that could be used and we can just
skip CAST5 and move on. Return NULL when CAST5 unsupported, just like
for other unsupported ciphers.
I'm going to make this one more generic than one library, and have it
support ciphers based on what libraries are available, so this more
generic name is appropriate.
