"If-None-Match:" header

For CGI programs that want to use "ETag" values (e.g. MD5 hashes) to
avoid re-sending dynamically-generated content that hasn't changed since
the last time the client requested it, it's useful to have access to the
HTTP header "If-None-Match".

This HTTP header isn't passed in the environment from Mathopd at
present.  I don't know the Mathopd source, but I'm guessing it should be
easy to patch Mathopd to provide it?  

Perhaps a more general solution would be useful, so that mathopd.conf
can say which HTTP headers we want to have passed to CGI programs:
possibly, all of them, at least for debugging...

Thanks,

- Martin.