internal/poll, os: cancel pending I/O when closing pipes on Windows

When closing a pipe, use CancelIoEx to cancel pending I/O.
This makes concurrent Read and Write calls return os.ErrClosed.

This change also enables some pipe tests on Windows.

Fixes #28477
Fixes #25835

Change-Id: If52bb7d80895763488a61632e4682a78336e8420
Reviewed-on: https://go-review.googlesource.com/c/go/+/164721


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>