Security Fix: Don't call chown() unless necessary.

Right now it's impossible to AppArmor NGINX when this module is enabled because you've got a mandatory `chown()` call. Worse yet, the nginx process exits if the `chown()` call fails. This means that you have to add `capability chown` to your NGINX AppArmor config. But granting chown to the application would undermine the security of the AppArmor sandbox. Hence this patch.

Here's a screenshot of NGINX not starting due to this bug:

![screen shot 2014-02-15 at 9 39 20 pm](https://f.cloud.github.com/assets/49262/2179576/6f3e12ee-96bc-11e3-90ed-2f6be890a03e.png)

If you're not familiar with AppArmor, this config file `/etc/apparmor.d/usr.local.nginx.sbin.nginx` should hopefully help you understand it better.

    #include <tunables/global>

    /usr/local/nginx/sbin/nginx {
      #include <abstractions/base>
      #include <abstractions/nameservice>
      #include <abstractions/openssl>
      #include <abstractions/user-tmp>

      capability dac_override,
      capability setgid,
      capability setuid,
      capability mknod,

      /etc/nginx/** r,
      owner /etc/ssl/** r,
      /home/nginx/** r,
      /home/ows/ows/occupywallst/occupywallst/media/** r,
      /home/recordings/** r,
      /home/soundboards/**.mp3 r,
      /home/soundboards/**.ogg r,
      /opt/celebcall/static/** r,
      /run/nginx.pid rw,
      /usr/local/nginx/conf/** r,
      /usr/local/nginx/html/** r,
      /usr/local/nginx/logs/** rw,
      owner /usr/local/nginx/proxy_temp/** rw,
      /usr/local/nginx/sbin/nginx mr,
      /var/cache/nginx/** rw,
      /var/cache/pagespeed/** rwlk,
      /var/log/pagespeed/** rw,
      /var/log/nginx/* w,
      /var/run/nginx.pid rw,
    }