Add SSLKEYLOGFILE support (nghttp client)

[jfhib]

I wanted to send a packet to the nghttp binary, and see it as wireshark.
For this purpose, SSLKEYLOGFILE environment variables were set, but nghttp did not log a keylog file.

#!/bin/bash

export SSLKEYLOGFILE="$HOME/sslkeylog.log"
nghttp -ans "url"

Can you officially add this feature?
The link is the source that I temporarily modified.

I referenced this pull request.

Thank you.