Added code to implement Dublin Core and Creative Commons RDF metadata.

Added code to implement Dublin Core and Creative Commons RDF metadata.

If these features are turned on (with global flags), a metadata link tag is
provided to the appropriate action ('dublincore' or 'creativecommons'). When
the wiki script receives these actions, it creates RDF files on the fly.

Also includes two valuable general functions: one to show an HTTP error
(used when the flags are turned off), and one to do content negotiation
(used to determine the MIME type for RDF files).

A helper function is also added to page output to display metadata links.