This fixes problems with setgid bits on the media directory

Seems like matching \b on unicode strings is unreliable across different
platforms (Debian). Using Unicode class lookahed/behinds seems to work
though.

This fixes a warning when setting $meta['relation']['firstimage']
using p_set_metadata.

In p_render() it is assumed that all renderers have a $doc variable

This includes a test for the minlen parameter for metadata keys

see http://irc.dokuwiki.org/index.php?d=2012-07-28#msg403531

This is a locale independent version of basename to work around
https://bugs.php.net/bug.php?id=37738

The function is not yet used anywhere. It should be at least used
where ever non-ASCII filenames and paths are handled. Simply replacing
all calls to basename() with this function might be the safest.