Distinguish between open/refused/timeout

[tmalaher]

In my local copy I've made modifications so that the portResult distinguishes between these three cases:
Connect OK - LISTEN process on the other end made a connection
Timeout - Firewall blocked connection
Connection Refused - Firewall did not block connection, but no LISTENing process on the other end. Thus this port could be used for a new server process

I find this handy because I don't want to have to open new ports in the firewall, and so I need to see what ports are open without having to run a listening process on all ports.

[JustinTimperio]

Send over a PR, and i'll give this new feature a test.

[tmalaher]

Not happy with my current implementation. Need to do some more research/experimentation.
Go does not make it easy to distinguish the reasons that a Dial fails, unlike ECONNREFUSED in C.