Re-factor the SGMLParser class to use the new markupbase.ParserBase class.

Re-factor the SGMLParser class to use the new markupbase.ParserBase class.
Use a new internal method, error(), consistently to raise parse errors;
the new base class also uses this.
Adjust the parse_comment() method to return the new offset into the buffer
instead of the number of characters scanned; this was the only helper
method that did it this way, so we have better consistency now.  Required
to share the new base class.
This fixes SF bug #448482 and #453706.