Improvements to label printing.

* [DCH_HTML,NEW] Added html_graph//0 which is like dcg_graph//0 for US-ASCII but excludes the HTML reserved characters.
* [GV_DCG,FIX] The HTML-like label parse was left-recursive.
* [GV_DCG,FIX] Use html_graph//0 ensures that tags are always parsed as such.
* [HTTP,FIX] Hopefully solved the irritating problem in which the Web front-end is unable to represent non-ASCII characters. We set the UTF-8 encoding explicitly in an HTTP header.
* [RDF_LIST,SC] Added rdf_list_name/2, coming from module RDF_NAME.
* [RDF_REIFICATION,EXT] Statement printing uses the new rdfs_label2/3, to ensure that RDF lists are printed correctly.
* [RDFS_LABEL,NEW] New module for RDFS label related predicates. Extracted from module RDFS_READ and added rdfs_label2/3 that returns labels for RDF terms based on preferred RDFS labels, and handles RDF lists specially.