New macro define-advice

* doc/lispref/functions.texi (Advising Named Functions): Document
define-advice.

* lisp/emacs-lisp/nadvice.el (define-advice): New macro.
* lisp/emacs-lisp/lisp-mode.el (lisp-imenu-generic-expression): Add
define-advice.
(lisp-font-lock-keywords-1): Add define-advice.