Support freenginx

[glyn]

Due to a disagreement among the NGINX developers, there is now the original NGINX project sponsored by the company F5 and a hard fork in the form of the freenginx project.

Although the nginx_robot_access module doesn't directly support freenginx, it probably wouldn't be too much work to achieve that since, at the time of writing, the APIs are similar. However, the ngx-rust module would first need to be upgraded to provide a Rust binding to freenginx.