nss_files: Consolidate file opening in __nss_files_fopen

Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

include/nss_files.h

@@ -0,0 +1,28 @@
+/* Internal routines for nss_files.
+   Copyright (C) 2020 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/>.  */
+
+#ifndef _NSS_FILES_H
+#define _NSS_FILES_H
+
+#include <stdio.h>
+
+/* Open PATH for reading, as a data source for nss_files.  */
+FILE *__nss_files_fopen (const char *path);
+libc_hidden_proto (__nss_files_fopen)
+
+#endif /* _NSS_FILES_H */