Difficult to get pipe handle

[funte]

The win32MessageBytePipe struct is unexported, its difficult to get the pipe handle, maybe should add this util function:

func UtilGetPipeHandle(f net.Conn) syscall.Handle {
	switch v := f.(type) {
	case *win32MessageBytePipe:
		return v.handle
	case *win32Pipe:
		return v.handle
	default:
		return 0
	}
}