lisp/simple.el (mail-encode-mml): New defvar.
lisp/mail/rmail.el (mail-encode-mml): Add a defvar.
(rmail-enable-mime-composing): Default to t.
(rmail-forward): Use MIME method of forwarding only if both
rmail-enable-mime-composing and rmail-enable-mime are non-nil.
Set mail-encode-mml non-nil if the MIME method was used.
lisp/mail/sendmail.el (mml-to-mime): Add autoload form.
(mail-encode-mml): Add a defvar.
(mail-mode): Make mail-encode-mml buffer-local and initialize it
to nil.
(mail-send): If mail-encode-mml is non-nil, run the outgoing
message through mml-to-mime, and reset mail-encode-mml to nil.