Use $server_name instead of $host in all sites/ paths to avoid broken URLs to static files for domain aliases in D7 (Nginx)

The Nginx configuration fragment included into sites (`includes/nginx_advanced_include.conf`) uses the `$host` variable in ways which are incompatible with site aliases. `$host` contains the `Host` header value, which is not necessarily the same as the name of the site directory.

location = /favicon.ico {
  access_log    off;
  log_not_found off;
  expires       30d;
  try_files     /sites/$host/files/favicon.ico $uri =204;
}

A site called "example.com" with an alias "example.org" will store its files in sites/example.com/files/. The existing configuration will serve requests for http://example.com/favicon.ico but not http://example.org/favicon.ico. I'm a newcomer to Nginx, but I believe that using `$server_name` instead of `$host` in these cases should resolve the problem.