Ensure that Warning headers are correctly handled as per RFC2616.

mod_cache: When serving from cache, only the last header of a multivalued header was taken into account. Fixed.

fix doxygen warnings

mod_cache: When a request other than GET or HEAD arrives, we must invalidate existing cache entities as per RFC2616 13.10. PR 15868.

Use ap_log_rerror() instead of ap_log_error() across mod_cache and mod_disk_cache.

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.

Remove the MOD_CACHE_REQUEST_REC hack, and pass the cache_request_rec structure through mod_cache's function parameters in the usual way.

Move private cache_* declarations out of the public mod_cache.h file.