Update from Gnulib

Make the following changes by hand, and run 'admin/merge-gnulib'.
* .gitignore: Add lib/malloc/*.gl.h.
* admin/merge-gnulib: Copy lib/af_alg.h and lib/save-cwd.h
directly from Gnulib, without worrying about Gnulib modules,
as these files are special cases.
(AVOIDED_MODULES): Remove malloc-posix.
* lib/malloc.c, lib/realloc.c, m4/malloc.m4, m4/realloc.m4:
* m4/year2038.m4: New files, copied from Gnulib.
* lib/malloca.c, lib/malloca.h:
* m4/close-stream.m4, m4/glibc21.m4, m4/malloca.m4:
Remove.  These are either no longer present in Gnulib, or are no
longer needed by modules that Emacs uses.
* oldXMenu/AddPane.c, oldXmenu/Addsel.c: Include XmenuInt.h first;
needed for new Gnulib.
* src/xmenu.c: Call emacs_abort, not abort.

lib/realloc.c

@@ -0,0 +1,63 @@
+/* realloc() function that is glibc compatible.
+
+   Copyright (C) 1997, 2003-2004, 2006-2007, 2009-2021 Free Software
+   Foundation, Inc.
+
+   This file is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   This file 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 Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* written by Jim Meyering and Bruno Haible */
+
+#include <config.h>
+
+#include <stdlib.h>
+
+#include <errno.h>
+
+#include "xalloc-oversized.h"
+
+/* Call the system's realloc below.  This file does not define
+   _GL_USE_STDLIB_ALLOC because it needs Gnulib's malloc if present.  */
+#undef realloc
+
+/* Change the size of an allocated block of memory P to N bytes,
+   with error checking.  If P is NULL, use malloc.  Otherwise if N is zero,
+   free P and return NULL.  */
+
+void *
+rpl_realloc (void *p, size_t n)
+{
+  if (p == NULL)
+    return malloc (n);
+
+  if (n == 0)
+    {
+      free (p);
+      return NULL;
+    }
+
+  if (xalloc_oversized (n, 1))
+    {
+      errno = ENOMEM;
+      return NULL;
+    }
+
+  void *result = realloc (p, n);
+
+#if !HAVE_MALLOC_POSIX
+  if (result == NULL)
+    errno = ENOMEM;
+#endif
+
+  return result;
+}