the hop-by-hop rules of rfc 2616 along with the entity response
rules. To make sure that they stay in sync; and to make it easier
to add (http) caching modules - this change moves them all into
one place (cache_util) and exposes a in-bound and out-bound
version to operate on the headers.

In short: we retire ap_cache_cacheable_hdrs_out() which was used
for both in- and out-put headers; and replace it by a single
ap_cache_cacheable_headers() which understands the hop-by-hop
rules. And then wrap this into an in- and out-put specific 
ap_cache_cacheable_headers_in()/out() which we can teach things
about entity responses and so on.. The latter which will also 
merge error and ensure content-type. 

This API change bumps up the minor MM by one.    



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@649162 13f79535-47bb-0310-9956-ffa450edef68