www/nginx-devel: Add NJS_QJS and NJS_XML options
The port pulled quickjs, libxml2 and libxslt unconditionally with NJS
and always built the QuickJS engine. Split this into the NJS_QJS and
NJS_XML options, so that www/nginx, www/nginx-devel and www/freenginx
offer the same set.
Sponsored by: Netzkommune GmbH