(mm-read-charset): Maybe use builtin.

(mm-read-charset): Maybe use builtin.
(mm-replace-chars-in-string): Maybe use subst-char-in-string.
(mm-multibyte-p, mm-with-unibyte-current-buffer)
(mm-with-unibyte): Use featurep, not string-match.
(mm-with-unibyte-buffer): Simplify.
(mm-quote-arg): Maybe use shell-quote-argument.