Also load for .sgm and .dtd files.

Also load for .sgm and .dtd files.
(sgml-specials, sgml-name-8bit-mode, sgml-char-names)
(sgml-font-lock-keywords, sgml-face-tag-alist, sgml-tag-face-alist)
(sgml-display-text, sgml-tag-alist, sgml-tag-help)
(sgml-auto-attributes): New variables.
(sgml-mode-common): New function.
(sgml-mode): Most code moved to it.
(sgml-name-char, sgml-name-self, sgml-maybe-name-self)
(sgml-name-8bit-mode, sgml-tag, sgml-attributes, sgml-auto-attributes)
(sgml-tag-help, sgml-maybe-end-tag, sgml-skip-tag-backward)
(sgml-skip-tag-forward, sgml-tags-invisible): New commands.
(sgml-beginning-of-tag, sgml-value): New functions.