[PATCH] variant Key negotiation on error pages

The Key: header being proposed for standardization replaces and extends
the Vary header to provide more fine-grained variant selection by caches.

The attached patch add the Key: header on Squid generated error pages so
that any receiving clients which support the new header are able to use
it for handling our error pages.

This also serves as a implementation example for how origin servers can
easily implement Key headers alongside Vary headers during the upgrade
transition period.
see http://tools.ietf.org/html/draft-fielding-http-key-02 for the
current specification.

Amos