* lisp/gnus/message.el: Give non-nil defaults for function vars

* lisp/gnus/message.el: Give non-nil defaults for function vars

Also prefer #' to quote functions.

(message-send-rename-function, message-reply-to-function)
(message-wide-reply-to-function, message-followup-to-function):
Use a non-nil default value so it can be used with `add-function`.
(message-do-send-housekeeping): Tweak accordingly.
(message-get-reply-headers): Simplify by η-reduction.