The attached patch does a couple of things to clean up Gnus's various
major modes:

- Creates a do-nothing `gnus-mode', derived from `special-mode', to
  serve as parent mode for all the other major modes. This isn't so
  important, but sometimes it would be nice to do (derived-mode-p
  'gnus-mode).
- Derives everything from `gnus-mode', and turns a couple of modes from
  functions that manually set up modes into proper calls to
  `define-derived-mode'.
- Removes code like "(setq buffer-read-only t)", as that's automatically
  done by `special-mode'.
- Where appropriate, deletes custom definitions for gnus-*-mode-hook, as
  these hooks are already automatically created by the define-mode
  calls. (Maybe this removal isn't worth doing?)

This patch probably needs more testing, but I'm floating it first for
any comments.

Eric

In GNU Emacs 27.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.1)
 of 2018-10-31 built on slip