Enable NPN

Next Protocol Negotiation extension. See https://technotes.googlecode.com/git/nextprotoneg.html

This should reduce round-trip time (RTT) for clients, thus giving faster load times.

This is essentially a micro-optimisation to try to turn on all the switches that can make things faster.
# Preview: /etc/nginx/sites-enabled/write.backdrop.conf

```
server {
  listen                *:80;

  server_name           write.backdrop;
  client_max_body_size 50m;
  proxy_set_header        X-Forwarded-Server  "$http_x_forwarded_server,$host";
  proxy_set_header        X-Forwarded-Host  "$http_x_forwarded_host,$host";
  proxy_set_header        Host $host;
    index  index.html index.htm index.php;

  access_log            /var/log/nginx/write.backdrop.access.log.json json_event;
  error_log             /var/log/nginx/write.backdrop.error.log;

  location / {
    proxy_pass          http://write.backdrop-vhost-upstream;
    proxy_read_timeout  90;
  }
  location /robots.txt {
    return 200 "User-agent: *\nDisallow: /";
  }
}
```

# VM: /etc/nginx/sites-enabled/write.backdrop.conf
```
server {
  listen                *:80;

  server_name           write.backdrop;
  client_max_body_size 50m;

  proxy_set_header        X-Forwarded-Server  "$http_x_forwarded_server,$host";
  proxy_set_header        X-Forwarded-Host  "$http_x_forwarded_host,$host";
  proxy_set_header        Host $host;
  add_header              Alternate-Protocol 443:npn-spdy/2;
    index  index.html index.htm index.php;

  access_log            /var/log/nginx/write.backdrop.access.log.json json_event;
  error_log             /var/log/nginx/write.backdrop.error.log;

  location /robots.txt {

    return 200 "User-agent: *\nDisallow: /";
  }
  location / {

    proxy_pass          http://write.backdrop-vhost-upstream;
    proxy_read_timeout  90;
    proxy_connect_timeout  90;
    proxy_redirect  off;
    proxy_set_header        Host $host;
    proxy_set_header        X-Real-IP $remote_addr;
    proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
  }
}
```