Support forward named pipe through SSH tunnel

[ud3v0id]

PowerShell/Win32-OpenSSH#435

Forward the remote Linux Unix domain socket to the local Windows named pipe:
ssh -NL //./pipe/docker_engine:/var/run/docker.sock user@host
The equivalent for a remote Windows named pipe would be:
ssh -NL //./pipe/docker_engine://./pipe/docker_engine user@host

Forward the local Windows named pipe to the remote Linux Unix domain socket:
ssh -NR /var/run/ssh-auth.sock://./pipe/ssh-pageant user@host
The equivalent for a remote Windows named pipe would be:
ssh -NR //./pipe/ssh-pageant://./pipe/ssh-pageant user@host

[msftclas]

All CLA requirements met.

[phxnsharp]

Thanks! This patch works for us.

It is worth noting that based on https://devblogs.microsoft.com/commandline/windowswsl-interop-with-af_unix/ this same code should allow for forwarding Windows Domain Sockets too, though it doesn't appear to work for me. Might require the insider SDK, which I have not installed.

[bagajjal]

@ud3v0id - Thank you for your time and efforts.
There are too many fundamental changes in this PR. I see several tests have failed.
Please add good amount of comments, test cases for your new code and address the test failures.

[bagajjal]

Closing this PR as there is no activity from the PR author.