Document negotiated cipher in HTTP log file

[triska]

With pull request SWI-Prolog/packages-ssl#86, ssl_session/2 also reports the negotiated cipher.

In my view, this information should be made available in log files, to gather statistics about the capabilities of clients that typically visit, so that suitable ciphers can be more easily chosen.

This of course affects exclusively HTTPS servers, where I think it would be great to have an additional cipher(Cipher) entry in the log file for each request. Please consider making this available by default in log files.

Thank you!