winnpc.c: add low-level connect_to_named_pipe() function.

This contains most of the guts of the previously monolithic function
new_named_pipe_client(), but it directly returns the HANDLE to the
opened pipe, or a string error message on failure.

new_named_pipe_client() is now a thin veneer on top of that, which
returns a Socket * by wrapping up the HANDLE into a HandleSocket or
the error message into an ErrorSocket as appropriate.

So it's now possible to connect to a named pipe, using all our usual
infrastructure (including in particular the ownership check of the
server, to defend against spoofing attacks), without having to have a
Socket-capable event loop in progress.