Add support for SSLKEYLOGFILE or equivalent

[pspacek]

Hello,

I think that supporting SSLKEYLOGFILE functionality as seen in NSS/Firefox and GnuTLS would make debugging weird HTTPS problems much simpler.

OpenSSL has support for it already, but it needs small tweaks on the apps side as well:

openssl/openssl#2343
openssl/openssl@4bf73e9

[daurnimator]

See #57

This issue is a luaossl concern and should be followed over at wahern/luaossl#150

[pspacek]

Sorry about the duplicate, I did not try hard enough.