Logjam vulnerability detection

[klyubin]

See https://weakdh.org.

• By eavesdropping on traffic, nogotofail should be able to detect whether a DH key exchange occurred over a group smaller than 1024 bit -- the current cutoff being deployed as mitigation against Logjam. The catch here is that clients which accept smaller groups will not be flagged unless the server actually uses a smaller group.
• By MiTMing traffic with servers which support export versions of DH cipher suites, nogotofail should be able to detect clients which accept the resulting smaller DH groups.