(sgml-xml-guess): Return the result rather than setting sgml-xml-mode.

(sgml-xml-guess): Return the result rather than setting sgml-xml-mode.
(sgml-mode, html-mode): Set sgml-xml-mode.
(sgml-skip-tag-backward): Tell if we skipped over matched tags.
(sgml-skip-tag-backward, sgml-electric-tag-pair-overlays): New var.
(sgml-electric-tag-pair-before-change-function)
(sgml-electric-tag-pair-flush-overlays): New functions.
(sgml-electric-tag-pair-mode): New minor mode.
(sgml-font-lock-keywords-2, sgml-get-context, sgml-unclosed-tag-p)
(sgml-calculate-indent): Use assoc-string.