(mime-decode-parameter-value): Decode MIME charset in multibyte buffer.

(mime-decode-parameter-value): Decode MIME charset in multibyte buffer.
(mime-decode-parameter-plist): Downcase attributes.
(mime-decode-parameters): Alias for `mime-decode-parameter-plist'
instead of `mime-decode-parameter-alist'.
Add autoload cookie.
(mime-parse-parameters-from-list): Make obsolete.
(mime-parse-parameters): Return results as a plist.
(mime-parse-Content-Type, mime-read-Content-Type): Move type check to the
caller side.
(mime-parse-Content-Disposition, mime-read-Content-Disposition):
Ditto.
(mime-parse-Content-Transfer-Encoding, mime-read-Content-Transfer-Encoding):
Ditto.