Support DH and ECDH key exchange in Ncat

[dmiller-nmap]

Unsure if this affects client connections, but ncat --ssl -l is certainly affected: http://seclists.org/nmap-dev/2016/q1/104

Related: #288

[isdaves]

Confirmed this is happening. Any chance to get ECDHE support in?

[dmiller-nmap]

Confirmed that this does not affect client connections, only listening connections.

[jeremyherbert]

any progress on this one ?