(mm-mime-mule-charset-alist): Require when compiling.

(mm-mime-mule-charset-alist): Require when compiling.
(mm-auto-save-coding-system): Prefer utf-8-emacs coding system to
emacs-mule.
(mm-find-mime-charset-region): Fix :mime-charset part.
(mm-mule-charset-to-mime-charset, mm-charset-to-coding-system)
(mm-mime-charset, mm-find-mime-charset-region): Look for
`:mime-charset' property of coding systems before `mime-charset'.
(mm-mule4-p, mm-enable-multibyte-mule4, mm-disable-multibyte-mule4)
(mm-with-unibyte-current-buffer-mule4): Deleted.
(mm-point-at-bol, mm-point-at-eol, mm-insert-byte)
(multibyte-char-to-unibyte): New.