(sgml-html-meta-auto-coding-function): New function.
(auto-coding-from-file-contents): Delete; merge functionality into
`set-auto-coding'.
(set-auto-coding): Move tests from `auto-coding-functions' so that
they have a lower priority than coding: tags. Put
`auto-coding-regexp-alist' tests before coding: tag tests.
(sgml-xml-auto-coding-function): Simply `intern' the match, and
test if it's a valid coding system.
(auto-coding-functions): Add `sgml-html-meta-auto-coding-function'.