MEDIUM: http: add a new "http-response" ruleset

Some actions were clearly missing to process response headers. This
patch adds a new "http-response" ruleset which provides the following
actions :
  - allow : stop evaluating http-response rules
  - deny : stop and reject the response with a 502
  - add-header : add a header in log-format mode
  - set-header : set a header in log-format mode
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 273fb8b..e6bc755 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -211,7 +211,8 @@
 		char *name;			/* default backend name during config parse */
 	} defbe;
 	struct list acl;                        /* ACL declared on this proxy */
-	struct list http_req_rules;		/* HTTP request rules: allow/deny/http-auth */
+	struct list http_req_rules;		/* HTTP request rules: allow/deny/... */
+	struct list http_res_rules;		/* HTTP response rules: allow/deny/... */
 	struct list block_cond;                 /* early blocking conditions (chained) */
 	struct list redirect_rules;             /* content redirecting rules (chained) */
 	struct list switching_rules;            /* content switching rules (chained) */