(sgml-at-indentation-p, sgml-tag)

(sgml-at-indentation-p, sgml-tag)
(sgml-parse-tag-name, sgml-looking-back-at, sgml-parse-tag-backward)
(sgml-inside-tag-p, sgml-get-context, sgml-show-context)
(sgml-insert-end-tag): New funs taken from xml-lite.el.
(sgml-calculate-indent): Use them.
(sgml-slash-matching): Rename from sgml-slash.
(sgml-slash): Copied from xml-lite and changed to use
sgml-slash-matching and sgml-quick-keys.