Don't let path.normalize get above the root.

Any path.join or path.normalize that starts with a / will not go "above" that after normalization.  This is important because /../foo is almost *always* some sort of error, and doesn't match the corollary in sh: `cd $p; pwd`

At the worse, this can be a vector for exploits, since a static file server might do path.join(docroot, path.normalize("/"+req)) to get the file.  If the normalized request path could be something like "/../../../etc/passwd" then bad things could happen.