modify go-socks5 to return the listener

[ainghazal]

From a review in #23:

I was thinking you could perhaps just call server.ListendAndServe with addr = {ip}:0 so as to make the OS pick the next available port for you.

I then saw, though, that the sock5 library you are using doesn't return the net.listener (https://github.com/armon/go-socks5/blob/master/socks5.go#L100) so if you actually need to know the picked port you have no way of extracting it :(

I guess we can leave this as-is for the time being, but it might be worth opening an issue as future work to open PR on https://github.com/armon/go-socks5 to return the listener (or set it as an attribute to the server struct).

• MIV-01-007 Possible DoS via Predictable Port Usage

Originally posted by @hellais in #23 (comment)