mod_cache: follow up to r1591328. Define the cache_merge_headers_out() function to merge r->err_headers_out into r->headers_out and add the ones from r->content_type/encoding if available. Use it in ap_cache_cacheable_headers_out() where the same is done and in cache_save_filter() where this has to be done before updating the entry.

mod_cache: Honour Cache-Control: no-store in a request.

mod_cache: Make sure Vary processing handles multivalued Vary headers and multivalued headers referred to via Vary.

mod_cache: Ignore response headers specified by no-cache=header and private=header as specified by RFC2616 14.9.1 What is Cacheable. Ensure that these headers are still processed when multiple Cache-Control headers are present in the response. PR 54706

The following now respects DefaultRuntimeDir/DEFAULT_REL_RUNTIMEDIR: - mod_cache: thundering herd lock directory

change signed single-bit fields to unsigned some of these were exposed to mods so the mmn is bumped, without regard to whether any compiler will have to generate different code

fix doxygen warnings

mod_cache: Make CacheEnable and CacheDisable configurable per directory in addition to per server, making them work from within a LocationMatch.

Fix some doxygen warnings

Optimise the configuration structures for mod_cache.

As cache_control_t is public, make ap_cache_control() public with it. Bump the MMN for addition of ap_cache_control(), and the addition of cache_control_t to the disk format.

Complete the optimisation of Cache-Control header parsing. Make the cache_control_t structure public so as to be available to mod_disk_cache.

Begin the process of optimising the parsing of Cache-Control headers. Parse the incoming Cache-Control and Pragma headers once, instead of on each test.

mod_cache: Optionally serve stale data when a revalidation returns a 5xx response, controlled by the CacheStaleOnError directive.

Remove the attempt to pass the cache key into the lock functions, use cache->key instead for this. Fixes a segfault caused when cache->key was populated, but the passed in key was not.

mod_cache: Allow control over the base URL of reverse proxied requests using the CacheKeyBaseURL directive, so that the cache key can be calculated from the endpoint URL instead of the server URL.

mod_cache: CacheLastModifiedFactor, CacheStoreNoStore, CacheStorePrivate, CacheStoreExpired, CacheIgnoreNoLastMod, CacheDefaultExpire, CacheMinExpire and CacheMaxExpire can be set per directory/location.

mod_cache: Add the cache_status hook to register the final cache decision hit/miss/revalidate. Add optional support for an X-Cache and/or an X-Cache-Detail header to add the cache status to the response. PR48241

Move mod_cache-private includes and defines from the public mod_cache.h interface into the private cache_util.h interface.

Make cache_provider_list and cache_request_rec private by moving them out of mod_cache.h.

Make cache_server_conf, cache_enable and cache_disable private. Remove public prefixes from ap_cache_accept_headers, ap_cache_try_lock and ap_cache_get_providers.

Make ap_cache_accept_headers, ap_cache_accept_headers, ap_cache_try_lock and ap_cache_check_freshness private.