Add auto-mode-alist for /config.php

[zonuexe]

refs #362

Currently, many php-mode users can not open config.php.
When php-mode is auto loaded, its priority is lower in auto-mode-alist
due to the append flag of add-to-list. Because of that, /config.php
matches conf-maybe-mode.

This patch is explicitly added to avoid conflict with the default
conf-maybe-mode in auto-mode-alist.

[ejmr]

Does this also match cfg.php? In my opinion this is an example of a regular expression becoming too complex; we have multiple regular expressions like this which I believe we should gradually replace. Even though it would be more verbose, we can and I think should rewrite this regular expression now to use (rx ...) so that it will be easier to understand exactly what we are matching.

Alternatively, or even together with (rx ...), I think it is better to use a separate call to add-to-list to cover the situation of config.php. Having a separate call like (add-to-list 'auto-mode-alist '("regex for config.php" . php-mode)) will create a tiny amount of code duplication---we don't need two calls to add-to-list---but in this scenario I believe it helps readability and maintainability.

[zonuexe]

This pattern is a modification of copying from files.el.

;; Original code: https://github.com/emacs-mirror/emacs/blob/65eee8392ff95f58f7b0bd036e1fe065523658c6/lisp/files.el#L2620
     ("[/.]c\\(?:on\\)?f\\(?:i?g\\)?\\(?:\\.[a-zA-Z0-9._-]+\\)?\\'" . conf-mode-maybe)

I think that rewriting this regexp pattern to rx is not appropriate.

[ejmr]

I apologize for the long delay but I finally had time to merge this tonight. Thank you for all your continued work.