RFC: avoid chroot() call if not changing root dir

[Pádraig Brady]

I was considering changing the coreutils tests from using
the internal setuidgid test utility to chroot --user...
to give more coverage for the latter.

However I noticed that chroot changes directory to /
even when we're just using it to change user IDs
(by specifying / as the "new" root).

So I was wondering about the attached to avoid the
chdir("/") in this case?  The chdir() is done to enhance
security a bit, but also for consistency. If we'd not
actually changing root dir then there are no security implications,
but I'm a bit worried about the consistency angle.
Now --user is a newish feature anyway so I'm not
that worried about breaking old scripts that assume
the implicit chdir("/").

thanks,
Pádraig.

chroot-nochdir.patch
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