Support for AF_UNIX on Windows

[gdamore]

Windows 10 now supports AF_UNIX. While we currently support named pipes, AF_UNIX has some distinct advantages, and so we should consider supporting it:

1. It is possible to use AF_UNIX to communicate with WSL processes on the same machine.
2. AF_UNIX preserves the UNIX semantics of binding and cleaning up the process, which could resolve an ongoing complaint.
3. AF_UNIX supports abstract sockets on both Windows and Linux

There are drawbacks as well:

• There is probably already an installed base of named pipe users
• Named pipes give us access to the security descriptor -- AF_UNIX probably won't

Long term it seems like AF_UNIX is going to be the way to go. What we'll probably do is add a unix: style URL, and maybe also an "mspipe:" URL. I imagine that in NNG 2.0 we would make ipc change from being mspipe to using AF_UNIX. In some further off future we would probably deprecate (remove) the mspipe URL altogether.