Check for links to see if update-rc.d service is enabled

This is on Ubuntu Precise (12.04), for a service managed with update-rc.d (a script in /etc/init.d/ and none in /etc/init/).
When running ``service: name=supervisor enabled=no``, it was always reporting changed=true. This fix changes the update-rc.d change reporting code, to check what kind of links update-rc.d said "already exist".