Process.handle was accessed without synchronization while wait() and
signal() could be called concurrently.

A first solution was to add a Mutex in Process but it was probably too
invasive given Process.handle is only used on Windows.

This version uses atomic operations to read the handle value. There is
still a race between isDone() and the value of the handle, but it only
leads to slightly incorrect error codes. The caller may get a:

  errors.New("os: process already finished")

instead of:

  syscall.EINVAL

which sounds harmless.

Fixes #9382

Change-Id: Iefcc687a1166d5961c8f27154647b9b15a0f748a
Reviewed-on: https://go-review.googlesource.com/9904Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>