Treat interpreter-mode-alist as alist of regexps, not literals

Treat interpreter-mode-alist as alist of regexps, not literals
Cf http://lists.gnu.org/archive/html/emacs-devel/2005-08/msg00472.html

* lisp/files.el (interpreter-mode-alist): Convert to regexps.
(set-auto-mode): Adapt for this.

* lisp/progmodes/cperl-mode.el (cperl-clobber-mode-lists):
Comment out unused variable.

* lisp/progmodes/cc-mode.el (interpreter-mode-alist):
* lisp/progmodes/python.el (interpreter-mode-alist):
* lisp/progmodes/ruby-mode.el (interpreter-mode-alist): Convert to regexps.

* lisp/progmodes/sh-script.el (sh-set-shell):
No longer use interpreter-mode-alist to get list of shells.

* etc/NEWS: Mention this.

Fixes: debbugs:15306