On Darwin and FreeBSD, supportsCloseOnExec is defined in its own file,
even though it is set to true as on other Unices. Drop the separate
definitions but keep the accompanying comments.

Change-Id: Iab1d20e1b2590800f141d54b55a099c9cd7ae57e
Reviewed-on: https://go-review.googlesource.com/97155
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>