socket-util: pipe for Windows.

socket-util: pipe for Windows.

Windows does have pipes (the interface is a little different).
We mostly use pipes in Linux to synchronize between parent and
children and also to handle fatal signals and then wake from poll_loop().

For Windows, we are using events for the same purpose. So don't
implement pipes for Windows.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>