(rmail-enable-multibyte): New variable

(rmail-enable-multibyte): New variable
(rmail): Bind coding-system-for-read to raw-text.  Set the buffer
multibyte if necessary.
(rmail-convert-file): Call rmail-decode-babyl-format only when we
don't have to call rmail-convert-to-babyl-format.
(rmail-decode-babyl-format): If rmail-file-coding-system is nil,
detect it.
(rmail-get-new-mail): Bind rmail-enable-multibyte locally.
(rmail-convert-to-babyl-format): Check the value of
rmail-enable-multibyte instead of enable-multibyte-characters.