bug#12632: file permissions checking mishandled when setuid

[Stefan Monnier]

>> 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.

        Stefan