MEDIUM: http: implement http-response track-sc* directive

This enables tracking of sticky counters from current response. The only
difference from "http-request track-sc" is the <key> sample expression
can only make use of samples in response (eg. res.*, status etc.) and
samples below Layer 6.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 0aa6643..9cdd589 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -242,10 +242,10 @@
 	return len;
 }
 
-/* for an http-request action ACT_HTTP_REQ_TRK_*, return a tracking index
+/* for an http-request/response action ACT_ACTION_TRK_SC*, return a tracking index
  * starting at zero for SC0. Unknown actions also return zero.
  */
-static inline int http_req_trk_idx(int trk_action)
+static inline int http_trk_idx(int trk_action)
 {
 	return trk_action - ACT_ACTION_TRK_SC0;
 }