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.

these are already checked in init.php, so we can save some space here