Updating license notice in AUCTeX files

[Arash Esbati]

Hi all,

now that AUCTeX is ELPA-only, I think we can/should also update the
license notice in the files.  IIUC, ELPA packages are considered as part
of Emacs, so the license part for all files will be:

--8<---------------cut here---------------start------------->8---
;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify it
;; under the terms of the GNU General Public License as published by the
;; Free Software Foundation, either version 3 of the License, or (at
;; your option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful, but
;; WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;; General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
--8<---------------cut here---------------end--------------->8---

Any comments?

Best, Arash