Fix grp.h endgrent, getgrent namespace (bug 18528)

[Joseph Myers]

On Fri, 12 Jun 2015, Roland McGrath wrote:

> That looks fine, but I think it would be nice if grp.h got a comment
> explaining how the visibility of these declarations has changed in the
> standards and in glibc.

Apart from a limited period where there was a bug in POSIX and that bug 
was reflected in glibc, the visibility *hasn't* changed; the group of 
three declarations was always meant to go together under the same 
conditions.  The details of past bugs seem like a distraction for normal 
readers of the header.

-- 
Joseph S. Myers
joseph@codesourcery.com