This patch rewrites export urls so that robots.txt can be used
to request that some (or all) export types are ignored by
robots when indexing the wiki.

For example:
User-agent: *
Disallow: _export/

or for example:
User-agent: *
Disallow: _export/raw/

Note: This rewriting is only done when $conf['userewrite'] is
set to '1' for using the rewrite rules from '.htaccess.dist'.

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