bug#12632: file permissions checking mishandled when setuid

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 22 Oct 2012 18:46:56 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: Eli Zaretskii <eliz@gnu.org>, 12632@debbugs.gnu.org
> 
> On 10/22/2012 05:40 PM, Stefan Monnier wrote:
> >>> We use IS_DIRECTORY_SEP in other places, so why avoid it here?
> >> > If it's not needed, it makes the code a bit harder to read.
> > I don't see why.  The name says very clearly what it is intended to do.
> > On the contrary, it seems more clear than comparing with '/' which only
> > makes sense if you happen to know that / is the special char used as
> > directory-separator.
> 
> Using IS_DIRECTORY_SEP indicates that it's necessary to
> process a file name using a system-dependent interpretation
> rather than the standard POSIX interpretation.  When
> plain X=='/' would do, it can mislead the reader if the code
> does IS_DIRECTORY_SEP (X), as the reader may naturally (and
> mistakenly) infer that the code was written that way because
> it must treat '\' specially on Windows.

Which is the case, so there's no misleading.

> It's primarily this clarity, not the brevity, that I was referring
> to.

This argument is a waste of everybody's time.  Feel free to leave the
test as it is, I will fix that after you commit the changeset.