Fix bug #9392 with rmail-forward.

Fix bug #9392 with rmail-forward.

 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.