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

[Roland McGrath]

> 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.

The name space controls altogether are a distraction for readers of the
header who are just going to use _GNU_SOURCE.  It's not "details of past
bugs".  It's details of past API versions that people may have written to.