bug#13125: Fix permissions bugs with setgid directories etc.

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 08 Dec 2012 23:26:27 -0800
> From: Paul Eggert <eggert@cs.ucla.edu>
> Cc: 13125@debbugs.gnu.org
> 
> On 12/08/2012 07:54 PM, Chong Yidong wrote:
> > Have you actually done a survey of all the callers to file-attributes to
> > see if this is true?
> 
> Yes, all the callers that are distributed as part of Emacs.

Did you find _any_ of them that even reference this attribute?  I
didn't, but perhaps I missed something.

Given the "wide" use, it is hard to reason what should be the value of
this attribute after these changes are installed.  You set them to
zero, which is neither nil nor t; thus, code that was testing for
either of these two values explicitly will now fail, while code that
was testing for non-nil will succeed where perhaps it shouldn't have.
For these reasons, I would suggest to leave the value at one of these
two, whichever is more frequent in real life.