Various good additions to DCG: e.g. RE-like predicate.

* [DBNL_GENERIC,SC] Rewrote dbnl_page//2 to a DCG rule. Colected dbnl_volume//2 and dbnl_title//2 as well. Simplifying existing predicates.
* [DCG_GENERIC,SC] Cleanup of predicate order, in categories now.
* [DCG_GENERIC,NEW] Added RE-like DCG predicates dcg_plus//1 and dcg_star//1. These cause ploblems in terms of backtracking if their DCGBody makes e.g. DB assertions that cannot be retracted while backtracking. This is of course a general DCG/Prolog issue, but it crops up here specifically.
* [HTML,NEW] Added html_attribute/2.