(rmail-view-buffer, rmail-enable-mime, rmail-show-mime-function,

(rmail-view-buffer, rmail-enable-mime, rmail-show-mime-function,
rmail-mime-feature): New variables to control MIME feature.
(rmail-file-coding-system): Initial value is nil.
(rmail, rmail-convert-file, rmail-insert-inbox-text): Check
rmail-enable-mime.  Read a file without any code conversion.
(rmail-variables): Setup local variables rmail-buffer and rmail-view-buffer.
(rmail-decode-babyl-format, rmail-convert-babyl-format): Perform code
conversion of RMAIL file if rmail-enable-mime is nil.
(rmail-show-message): Make sure to be in rmail-buffer.  If
rmail-enable-mime is t, call appropriate function to decode MIME
format.
(rmail-mail, rmail-reply): Call rmail-start-mail with argument
rmail-view-buffer.