WIP: Remote Dynamic Forwarding

[darinkes]

This is still WIP and in RFC state.

It implements reverse dynamic forwarding added in OpenSSH 7.5.

https://www.openssh.com/releasenotes.html

• ssh(1): add support for reverse dynamic forwarding. In this mode,
  ssh will act as a SOCKS4/5 proxy and forward connections
  to destinations requested by the remote SOCKS client. This mode
  is requested using extended syntax for the -R and RemoteForward
  options and, because it is implemented solely at the client,
  does not require the server be updated to be supported.

Usage is the same like in the ssh(1) commandline tool (https://man.openbsd.org/ssh.1)

using (var client = new SshClient("localhost", 22, "username", "password"))
{
    client.Connect();
    var forwarding = new ForwardedPortRemote("127.0.0.1", 5555);
    client.AddForwardedPort(forwarding);

    forwarding.Start();
}

[WojciechNagorski]

@darinkes PR needs refreshing.