Provide new gnus-mode, derive all gnus major modes from this

Provide new gnus-mode, derive all gnus major modes from this

* lisp/gnus/gnus.el (gnus-mode): New do-nothing major mode, derived
  from special mode.
* lisp/gnus/gnus-sum.el (gnus-summary-mode): Change from a function to
  a major mode, derive from gnus-mode.
  (gnus-summary-setup-buffer): Change call a bit -- can no longer pass
  an argument to the mode function.
* lisp/gnus/gnus-srvr.el (gnus-browse-mode): Derive from gnus-mode.
  (gnus-server-setup-buffer): Remove unnecessary function.
  (gnus-enter-server-buffer): Call gnus-server-mode here, and call it
  whether the server buffer already existed or not.
  (gnus-server-mode): Change from a function to a major mode.
  (gnus-server-mode-hook): Delete custom option, this is automatically
  created.
* lisp/gnus/gnus-salt.el (gnus-tree-mode): Derive from gnus-mode.
  (gnus-tree-mode-hook): Delete custom option, this is automatically
  created.
* lisp/gnus/gnus-kill.el (gnus-kill-file-mode-hook): Delete custom
  option.
* lisp/gnus/gnus-group.el (gnus-group-mode):
* lisp/gnus/gnus-art.el (gnus-article-mode):
* lisp/gnus/gnus-agent.el (gnus-category-mode): Derive from gnus-mode.
  (gnus-category-mode-hook): Delete custom option.

(Bug#33263)