Add :after-hook facility to define-derived-mode.

Add :after-hook facility to define-derived-mode.

This allow a form to be evaluated _after_ a major mode's hooks have been run.
It is needed to solve some problems in CC Mode, including bug #16759 and
bug #23476.

* lisp/emacs-lisp/derived.el (define-derived-mode): introduce the new argument
`:after-hook', and generate the requisite code for it.
(derived-mode-make-docstring): Take account of the possibility of :after-hook.

* lisp/subr.el (delayed-after-hook-forms): New variable.
(run-mode-hooks): As the last thing evaluate the forms in
delayed-after-hook-forms.

* doc/lispref/modes.texi (Derived Modes): Document :after-hook.
(Mode Hooks): Document the new feature in run-mode-hooks.

* etc/NEWS: Note the new feature.