As suggested by Brad in CL 83275.

From what I can tell getsockopt syscall on these OSes doesn't return
ERANGE in any case, so these versions don't include the buffer growing.

Change-Id: I667c3751021e26defec0a04761a689fa4b78c65d
Reviewed-on: https://go-review.googlesource.com/84016
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>