hurd: compliance fixes for getgroups

Fail with EINVAL when the requested number of groups is negative,
or when it is positive but less than the actual number of groups.

2012-04-27  Pino Toscano  <toscano.pino@tiscali.it>

	* sysdeps/mach/hurd/getgroups.c (__getgroups): Return -1 and set EINVAL
	for negative `n' or less than `ngids'.
--- a/sysdeps/mach/hurd/getgroups.c
+++ b/sysdeps/mach/hurd/getgroups.c
@@ -30,6 +30,9 @@ __getgroups (n, gidset)
   int ngids;
   void *crit;
 
+  if (n < 0)
+    return __hurd_fail (EINVAL);
+
   crit = _hurd_critical_section_lock ();
   __mutex_lock (&_hurd_id.lock);
 
@@ -53,7 +56,7 @@ __getgroups (n, gidset)
       /* Now that the lock is released, we can safely copy the
 	 group set into the user's array, which might fault.  */
       if (ngids > n)
-	ngids = n;
+	return __hurd_fail (EINVAL);
       memcpy (gidset, gids, ngids * sizeof (gid_t));
     }
   else