Make Message respect header removal instructions more

Make Message respect header removal instructions more

* doc/misc/message.texi (Forwarding): Document it.

* lisp/gnus/message.el (message-forward-ignored-headers): Improve
documentation.
(message-forward-included-headers): Ditto.
(message-forward-included-mime-headers): New user option.
(message-remove-ignored-headers): Use it to preserve the necessary
MIME headers.
(message-forward-make-body): Remove headers when forwarding as
MIME, too.