Optionally log master secrets for TLS connections

[jsha]

Sometimes it's necessary to decrypt your own TLS connections to debug their contents. Wireshark supports this quite nicely with its decryption feature. For non-DH key agreement, you simply provide the private key of the server. However, for DH key agreement, or when you are acting only as a client, that doesn't work. Firefox and Chrome support the environment variable SSLKEYLOGFILE to write the master secrets used to a file, for decryption by Wireshark. It would be great to support this or a similar mechanism for logging master secrets in Node.

Key log format: https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format
Helpful Stack Exchange howto: https://security.stackexchange.com/questions/35639/decrypting-tls-in-wireshark-when-using-dhe-rsa-ciphersuites/42350#42350
Wireshark decryption docs: https://wiki.wireshark.org/SSL

[jasnell]

@jsha ... can you open this against either nodejs/io.js or nodejs/node instead? It's not likely that this would land in v0.10 or v0.12 here.

[jasnell]

See: nodejs/node#59