www/freenginx: fix rc script required_files path

Use %%ETCDIR%%/nginx.conf in the rc script instead of the
obsolete hardcoded %%PREFIX%%/etc/nginx/freenginx.conf path.

The port is configured with:

--prefix=/usr/local/etc/freenginx
--conf-path=/usr/local/etc/freenginx/nginx.conf

The previous required_files setting no longer matched the
actual configuration file location and could prevent the
service from starting after installation or upgrade.

PR: 290461
Sponsored by: Netzkommune GmbH