Several small fixes.
1. RDF_SERIAL is a mess now, consider cleanup/rewrite.

* [RDF_SERIAL,EXT] Added rdf_load2/4 as a public predicate, for setting the serialization format explicitly.
* [RDF_SERIAL,NEW] Added rdf_serialization_format/2. Not used yet.
* [RDF_SERIAL,EXT] Allow the file type to differ from the serialization format.
* [RDF_TMS,:-(] Commented out the lines that require bnode2literal mappings.