Making apache server-status optional with a marker

Apache server status won't be enable by default. User will have to add `public_server_status` marker to enable it. This will also apply on the `ExtendedStatus` option. 

Based on Trello card:
https://trello.com/c/R0H10FIH/22-make-server-status-user-configurable-via-marker
I will rename this to `write_server_status`, the function name should reflect what it does.