Imported latest 6.x version (r493) from SVN development repository.

Changelog:
- Updated ARC2 compatibility (now works with release 2008-05-30).
- Fixed white-screening bug when exporting RDF data (#261237, #242509, #243971).
- Fixed repository administration menu paths.
- Implemented unit tests for the RDF repository create/rename/delete APIs.
- Implemented unit tests for the RDF statement count/insert/query/delete APIs.
- Added a bunch of the FOAF terms to the predefined, auto-generated convenience classes.
- Clear the schema cache in rdf_db_create_repository().
- Implemented hook_schema_alter() for defining the schema for user-created repository tables.
- Added new API functions rdf_get_type() and theme_rdf_value().
- Simplified QName/CURIE creation by implementing a $rdf_namespaces global.
- Merged the RDF DB hook implementations with the existing RDF API handlers.
- Bug fix for rdf_count().
- Optimized rdf_literal() to not construct an RDF_Literal object for plain non-tagged string literals.
- Allowed non-wrapped object literals in rdf_denormalize() for greater convenience.