Bigger patch for (partial) tinylogin integration

- Made a new dir loginutils
- Moved all applets from pwd_grp to loginutils
- Added new applets su.c login.c to loginutils
- Made a new dir libpwdgrp
- Moved everything from pwd_grp/libpwd_grp there
- Added shadow.c to libpwdgrp
- Removed dir pwd_grp
- Added usage strings for login and su to usage.h
- Changed main Makefile to reflect the dir rearrangements
[Parts of this patch may overlap with my other two patches]

libpwdgrp/grent.c

@@ -0,0 +1,54 @@
+/*
+ * grent.c - This file is part of the libc-8086/grp package for ELKS,
+ * Copyright (C) 1995, 1996 Nat Friedman <ndf@linux.mit.edu>.
+ * 
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Library General Public
+ *  License as published by the Free Software Foundation; either
+ *  version 2 of the License, or (at your option) any later version.
+ *
+ *  This library is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  Library General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Library General Public
+ *  License along with this library; if not, write to the Free
+ *  Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ *
+ */
+
+/*
+ * setgrent(), endgrent(), and getgrent() are mutually-dependent functions,
+ * so they are all included in the same object file, and thus all linked
+ * in together.
+ */
+
+#include "busybox.h" 
+
+#include <unistd.h>
+#include <fcntl.h>
+#include "grp.h"
+
+static int grp_fd = -1;
+
+void setgrent(void)
+{
+	if (grp_fd != -1)
+		close(grp_fd);
+	grp_fd = open("/etc/group", O_RDONLY);
+}
+
+void endgrent(void)
+{
+	if (grp_fd != -1)
+		close(grp_fd);
+	grp_fd = -1;
+}
+
+struct group *getgrent(void)
+{
+	if (grp_fd == -1)
+		return NULL;
+	return __getgrent(grp_fd);
+}