Extend MaxAgeCheck to support http_cache_control

Code looks good to me.

Approach looks good to me: as far as I understand, Purge is explicitly meant to cover "shared caches", so whenever s-maxage header is present it can ignore cache.page.max_age.

Tested: warning disappears from the status report when it shouldn't be there.

The change in logic isn't big so I don't think this needs test coverage?

Hmm I think a more elegant option here would be to throw an event, allowing the max_age variable to be changed. What if other modules have opinions here?