mtn sync file: and
mtn sync ssh: do not work reliably on Windows MinGW.
The core problem is that Win32 does not support
select on pipes.
This branch attempts to replace Win32 pipes by sockets.
It fails, because ssh doesn't create sockets when it runs mtn.
See comments in netxx_pipe.hh
See nvm.experimental.win32 pipes 2 for another approach.
Status: Stalled; use Cygwin instead, where things just work.