CL 28484 mistakenly assumed that WSARecv returns WSAEINPROGRESS
when there is nothing to read. But the error is ERROR_IO_PENDING.
Fix that mistake.

I was about to write a test for it. But I have found
TestTCPReadWriteAllocs in net package that does nearly what I need,
but was conveniently disabled. So enable and extend the test.

Fixes #16988

Change-Id: I55e5cf8998a9cf29e92b398d702280bdf7d6fc85
Reviewed-on: https://go-review.googlesource.com/28990
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>