Added indentation to directive/variables indexes. While these files aren't designed to be read/edited by humans, indentation makes things more convenient. E.g., terminal is no longer flooded by a very long line with full list of directives if dirindex.xml is matched by grep.

<!--
  Copyright (C) Igor Sysoev
  Copyright (C) Nginx, Inc.
  -->

<!DOCTYPE error SYSTEM "../dtd/error.dtd">

<error name="404: not found" link="404.html" lang="en" />