src: accept passphrase when crypto signing with private key

Previous behaviour was to drop to an openssl prompt
("Enter PEM pass phrase:") when supplying a private key with a
passphrase. This change adds a fourth, optional, paramter that
will be used as the passphrase.
To include this parameter in a backwards compatible way it was
necessary to expose the previously undocumented (and unexposed)
feature of being able to explitly setting the output encoding.