1. Split off from RDF_NAMESPACE an XML_NAMESPACE subset.
2. Several RDF-related fixes.

* [DBPEDIA,SC] Removed find_dbpedia_agent/4 which is STCN-specific. Relocated to the STCN project.
* [RDF_EXPORT,FIX] URIs that do not have a registered namespace would not be named.
* [RDF_SERIAL,FIX] In rdf_save, the determination of the internal format name based on the serialization format name was broken.
* [RDF_SERIAL,SC] In rdf_save, the namespaces are no longer explicitly supplied.
* [XML,XMLS,SC] Relocated to XML subdirectory.
* [XML_NAMESPACE,NEW] New module, extracting the XML namespace part out of module RDF_NAMESPACE.