rewrite export URLs

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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