Always append function to message-exit-actions

Probably safest to have our return/restore functions at the end of the
message-exit-actions list.

* lisp/gnorb-gnus.el (gnorb-gnus-check-outgoing-headers,
  gnorb-gnus-outgoing-do-todo): Use the 't argument to add-to-list.

* lisp/nngnorb.el (gnorb-summary-reply-hook): Ditto.