Various fixes for RDF support, mostly RDF tabular related.

* [IANA_TO_RDF,FIX] Superfluous module imports.
* [RDF_FILE,RDF_FILE_DB,FIX] To avoid module load cross-dependencies, we store very basic RDF file info in RDF_FILE_DB.
* [RDF_LIST,FIX] rdf_is_list/1 should check for (+) instantiation.
* [RDF_META.SC] Superfluous module imports.
* [RDF_NAME,FIX] Graph would get instantiated to the empty atom when it should stay uninstantiated in rdf_graph_name//1.
* [RDF_NTRIPLES_WRITE,FIX] Blank nodes (non-replaced) has a superfluous forward slash in their name.
* [RDF_NTRIPLES_WRITE,SC] IRI-fied blank nodes now end their prefix in #, not /.
* [RDF_SERIAL,FIX] Format option for rdf_save/3 got broken along the way...
* [RDF_TERM_HTML,EXT] Add an anchor link for graphs whose name is a URL.
* [RDFS_LABEL_EXT,FIX] rdf_label/4 now also returns non-language tagged strings.
* [URI_SCHEME,FIX] URI scheme look up synched to new RDF API.