fixed auto-mode-alist

[chillum]

This code:

(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.