BUG/MINOR: cache: reduce the max-age lifetime by the response's Age

BUG/MINOR: cache: reduce the max-age lifetime by the response's Age

A response received with "Cache-Control: max-age=3" and "Age: 2" is kept
fresh in the cache for 3 seconds, and delivered with an Age reaching 4 or
5 seconds, while it only had one second of freshness left. In general,
responses coming through upstream caches are kept fresh for as long as
their Age in excess, at most the cache's max-age, which delivers stale
contents to clients, against RFC 9111#4.2. Clients may see outdated
versions of resources for longer than what the origin server specified.

The expiration date of an entry is computed as the reception date plus
the lifetime returned by http_calc_maxage(), capped by the cache's
max-age. The Age header is parsed afterwards, only to refuse responses
whose Age already exceeds their lifetime, and to compute the Age header of
the copies. But a lifetime given by max-age or s-maxage counts from the
generation of the response at the origin, so the time already spent in
upstream caches, given by Age, must be deducted from it. A lifetime given
by Expires is an absolute date and must not be reduced.

http_calc_maxage() now also reports through a new argument whether the
lifetime comes from s-maxage or max-age, and in this case the effective
lifetime is reduced to what remains after deducing the Age.

This patch may be backported to all supported versions.