The Ctty field is a child descriptor number when Setctty is set,
but a parent descriptor when Foreground is set. This is absurd
but changing either behavior breaks existing programs.
With this change we at least document how it works.
For #29458
Change-Id: If9cf0a1a1e6ed0d4a4edae5043016d5b4ee3308b
Reviewed-on: https://go-review.googlesource.com/c/go/+/229768
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Damien Neil <dneil@google.com>