os: handle the sticky bit separately for *BSD and Solaris

open(2) and mkdir(2) won't set the sticky bit on *BSD and Solaris.
This behavior is mentioned on sticky(8).
see also: https://github.com/dotcloud/docker/pull/6587

Fixes #8383.

Change-Id: Ic4733700f9926b9fc2b6fd1f998acec34e518764
Reviewed-on: https://go-review.googlesource.com/1673
Reviewed-by: Ian Lance Taylor <iant@golang.org>