nss: Add __nss_fgetent_r

And helper functions __nss_readline, __nss_readline_seek,
 __nss_parse_line_result.

This consolidates common code for handling overlong lines and
parse files.  Use the new functionality in internal_getent
in nss/nss_files/files-XXX.c.

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

nss/nss_parse_line_result.c

@@ -0,0 +1,46 @@
+/* Implementation of __nss_parse_line_result.
+   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/>.  */
+
+#include <nss_files.h>
+
+#include <assert.h>
+#include <errno.h>
+
+int
+__nss_parse_line_result (FILE *fp, off64_t offset, int parse_line_result)
+{
+  assert (parse_line_result >= -1 && parse_line_result <= 1);
+
+  switch (__builtin_expect (parse_line_result, 1))
+    {
+    case 1:
+      /* Sucess.  */
+      return 0;
+    case 0:
+      /* Parse error.  */
+      __set_errno (EINVAL);
+      return EINVAL;
+    case -1:
+      /* Out of buffer space.  */
+      return __nss_readline_seek (fp, offset);
+
+      default:
+        __builtin_unreachable ();
+    }
+}
+libc_hidden_def (__nss_parse_line_result)