This change adds support for message mode pipes and uses them to support
CloseWrite() to better match TCP and UNIX sockets. Message mode pipes
support writing and (optionally) reading data in message-sized chunks.
This is useful for us because when in this mode a zero-sized message can
be read. We use this zero-sized message to signal that no more writes will
arrive. This is not standard practice in Windows, but it is a reasonable
compromise.
This includes Reader, Writer, and Closer interfaces on top of
Windows file handles in a way that does not block the system thread
while the IO is outstanding.
This also includes net.Listener and net.Conn for named pipes.
John Starks