Fix a bug by not affecting c-default-style

Previously we used the code

    (add-to-list 'c-default-style '(php-mode . "php"))

to automatically use PHP Mode’s default style of indentation and
formatting.  However, this can conflict with existing settings in a
way that stops PHP Mode from working.  For example, if a user has

    (setq c-default-style "linux")

in his or her Emacs config file, then our use of `add-to-list' above
can result in malformed value for `c-default-style', e.g.

    ((php-mode . "php") . "linux")

This patch addresses the problem by first removing our modification to
`c-default-style'.  We then add `php-enable-default-coding-style',
similar to functions like `php-enable-drupal-coding-style'.  Like the
previous styles, the default is now selectable via `M-x customize-group
<RET> php-mode'.  And finally we update the README to explain how the
user can set the default style via either customize-group or a hook.

GitHub-Issue: 209