PATCH: http_reply_access

This patch came out of the work I have been doing on client_side in the
rbcollins_filters branch. It's not actually _part_ of that work.. I just
figured out how and where to do it there :-]

It implements a new acl type rep_mime_type, complementary to
req_mime_type, and a new access list http_reply_access which allows acls
to be applied to replies. The prime target is content-type filtering.
One such example would be to only allow a particular set of users the
ability to download tgz. (ie by combining proxy_auth and rep_mime_type).

I created a new acl type rather than moving the request vs reply header
field decision to the calling routine as that seems to be the approach
all over the place.

Rob