Detect etag supercookies

[Etags](https://en.wikipedia.org/wiki/HTTP_ETag) can be [used as supercookies](https://samy.pl/evercookie/). Detecting them is not totally trivial, because they are a multi-purpose technology that can be used innocently, but the fact that Nginx and Apache have standard recipes for setting them means that we should in principle be able to detect and act upon their non-standard uses.