nss: Access nss_files through direct references

This partially fixes static-only NSS support (bug 27959): The files
module no longer needs dlopen.  Support for the dns module remains
to be added, and also support for disabling dlopen altogether.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

nss/nss_files_functions.c

@@ -0,0 +1,43 @@
+/* Direct access for nss_files functions for NSS module loading.
+   Copyright (C) 2021 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 <nss_module.h>
+#include <nss_files.h>
+
+void
+__nss_files_functions (nss_module_functions_untyped pointers)
+{
+  void **fptr = pointers;
+
+  /* Functions which are not implemented.  */
+#define _nss_files_getcanonname_r NULL
+#define _nss_files_gethostbyaddr2_r NULL
+#define _nss_files_getpublickey NULL
+#define _nss_files_getsecretkey NULL
+#define _nss_files_netname2user NULL
+
+#undef DEFINE_NSS_FUNCTION
+#define DEFINE_NSS_FUNCTION(x) *fptr++ = _nss_files_##x;
+#include "function.def"
+
+#ifdef PTR_MANGLE
+  void **end = fptr;
+  for (fptr = pointers; fptr != end; ++fptr)
+    PTR_MANGLE (*fptr);
+#endif
+}