Due to parallel development of javadoc and doclint we have two similar (but different) copies of HtmlTag in langtools. The one for doclint is primarily for inbound processing and checking; the one for javadoc is primarily for outbound tree construction. These could reasonably be merged. Ideally, these might be in a HTML-specific package, but since is is only a few files, it is enough for javadoc to reference the merged version in doclint.

This cleanup also affects/includes corresponding updates to HtmlTag.Attr, HtmlVersion.

Finally, the naming of the members for HtmlTag.AttrKind is less than ideal, and this is an opportunity to rationalize that.