fixed auto-mode-alist

This code:

```lisp
(add-to-list 'auto-mode-alist
 '("nginx\.conf$" . nginx-mode)
 '("/etc/nginx/.*" . nginx-mode))
```

was broken in two ways:

1. wrong regexps
2. wrong add-to-list syntax: the third argument is append, not an additional item.

Changed the `auto-mode-alist` part so it works now, but only for `*/nginx/*.conf` and `nginx.conf`.

Sites-enabled stuff should be disabled by default: clashes with `apache-mode` (mentioned this in docs).

Adding of all files under `/etc/nginx` is evil (they're not necessarily nginx configs actually), so disabled that as well.