Use define-minor-mode in Gnus where applicable.

Use define-minor-mode in Gnus where applicable.
* mml.el (mml-mode): Use define-minor-mode.
* gnus-undo.el (gnus-undo-mode-map): Initialize in declaration.
(gnus-undo-mode): Use define-minor-mode.
* gnus-sum.el (gnus-dead-summary-mode-map): Initialize in declaration.
(gnus-dead-summary-mode): Use define-minor-mode.
* gnus-salt.el (gnus-pick-mode-map, gnus-binary-mode-map):
Initialize in declaration.
(gnus-pick-mode, gnus-binary-mode): Use define-minor-mode.
* gnus-ml.el (gnus-mailing-list-mode-map): Initialize in declaration.
(gnus-mailing-list-mode): Use define-minor-mode.
* gnus-draft.el (gnus-draft-mode-map): Initialize in declaration.
(gnus-draft-mode): Use define-minor-mode.
* gnus-dired.el (gnus-dired-mode-map): Initialize in declaration.
(gnus-dired-mode): Use define-minor-mode.