Purpose: Convert logical URI to physical path.
Perform operations on logical path (ppath in rq->vars) to convert it into a full local file system path.
Return REQ_PROCEED if ppath in rq->vars contains the full local file system path, or REQ_NOACTION if not.
To redirect the client to another site, change ppath in rq->vars to /URL. Add url to rq->vars with full URL (for example, http://home.netscape.com/). Return REQ_PROCEED.