net: make windows (*netFD).connect work like its unix version

CL 17821 used syscall.CancelIoEx to cancel outstanding connect
call, but did not check for syscall.CancelIoEx return value.
Also I am worried about introducing race here. We should use
proper tools available for us instead. For example, we could
use fd.setWriteDeadline just like unix version does. Do that.

Change-Id: Idb9a03c8c249278ce3e2a4c49cc32445d4c7b065
Reviewed-on: https://go-review.googlesource.com/17920Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>

src/net/fd_windows.go

@@ -357,15 +357,9 @@ func (fd *netFD) connect(la, ra syscall.Sockaddr, deadline time.Time, cancel <-c
 		go func() {
 			select {
 			case <-cancel:
-				// TODO(bradfitz,brainman): cancel the dial operation
-				// somehow. Brad doesn't know Windows but is going to
-				// try this:
-				if canCancelIO {
-					syscall.CancelIoEx(o.fd.sysfd, &o.o)
-				} else {
-					wsrv.req <- ioSrvReq{o, nil}
-					<-o.errc
-				}
+				// Force the runtime's poller to immediately give
+				// up waiting for writability.
+				fd.setWriteDeadline(aLongTimeAgo)
 			case <-done:
 			}
 		}()