Imported latest 6.x version (r462) from SVN development repository.

Changelog:
- Folded the RDF DB, RDF Export and RDF Import submodules back into the base module to allow for easier dependency management.
- Updated menu routing for Drupal 6.2 compatibility (#264076).
- Disabled unavailable formats when ARC2 is not installed.
- Updated documentation; added sponsors to README.txt.