* lisp/emacs-lisp/cl-generic.el: Add (major-mode MODE) context

* lisp/emacs-lisp/cl-generic.el: Add (major-mode MODE) context

(cl--generic-derived-specializers): New function.
(cl--generic-derived-generalizer): New generalizer.
(cl-generic-generalizers): New specializer (derived-mode MODE).
(cl--generic-split-args): Apply the rewriter, if any.
(cl-generic-define-context-rewriter): New macro.
(major-mode): Use it to define a new context-rewriter, so we can write
`(major-mode MODE)' instead of `(major-mode (derived-mode MODE))'.

* lisp/frame.el (window-system): New context-rewriter so we can write
`(window-system VAL)' instead of (window-system (eql VAL)).
(cl--generic-split-args): Apply the rewriter, if any.
(frame-creation-function): Use the new syntax.

* lisp/term/x-win.el (window-system-initialization)
(handle-args-function, frame-creation-function)
(gui-backend-set-selection, gui-backend-selection-owner-p)
(gui-backend-selection-exists-p, gui-backend-get-selection):
* lisp/term/w32-win.el (window-system-initialization)
(handle-args-function, frame-creation-function)
(gui-backend-set-selection, gui-backend-get-selection)
(gui-backend-selection-owner-p, gui-backend-selection-exists-p):
* lisp/term/pc-win.el (gui-backend-get-selection)
(gui-backend-selection-exists-p, gui-backend-selection-owner-p)
(gui-backend-set-selection, window-system-initialization)
(frame-creation-function, handle-args-function):
* lisp/term/ns-win.el (window-system-initialization)
(handle-args-function, frame-creation-function)
(gui-backend-set-selection, gui-backend-selection-exists-p)
(gui-backend-get-selection):
* lisp/startup.el (handle-args-function):
* lisp/term/xterm.el (gui-backend-get-selection)
(gui-backend-set-selection): Use the new syntax.