windows version of Pipe function is implemented by calling
syscall.Pipe which returns handles inheritable by client process,
and then adjusting returned handles with syscall.CloseOnExec.

Just create non-inheritable handles in the first place.
Now that we don't have a race window in the code, drop use
of syscall.ForkLock.

Change-Id: Ie325da7c2397b5995db4a5ddb0117e2ce1745187
Reviewed-on: https://go-review.googlesource.com/72010Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>