support: Add <support/xdirent.h>

Use static functions for readdir/readdir_r, so that
-D_FILE_OFFSET_BITS=64 does not improperly redirect calls to the wrong
implementation.

Reviewed-by: DJ Delorie <dj@redhat.com>

support/support_readdir_r_check.c

@@ -0,0 +1,35 @@
+/* Error-checking helper for xreaddir_r, xreaddir64_r.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library 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.
+
+   The GNU C 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/xdirent.h>
+
+#include <support/check.h>
+
+int
+support_readdir_r_check (const char *name, int result, void *buf, void *ptr)
+{
+  if (result != 0)
+    {
+      errno = result;
+      FAIL_EXIT1 ("%s: %m", name);
+    }
+  if (buf != ptr)
+    FAIL_EXIT1 ("%s: buffer pointer and returned pointer differ: %p != %p",
+                name, buf, ptr);
+  return result;
+}