add support for r-maxage

r-maxage is a new directive that takes precendence over s-maxage and
max-age.

While s-maxage is honored by any shared cache, r-maxage is only taken
into account by reverse proxy caches, and thus ignored by proxy and
private caches. Setting r-maxage allows applications to ensure the
response is only cached by their reverse cache, and thus are able in
particular to purge the cache knowing that it will not remain stale in
intermediate proxy servers.