[MINOR] frontend: count denied TCP requests separately

It's very disturbing to see the "denied req" counter increase without
any other session counter moving. In fact, we can't count a rejected
TCP connection as "denied req" as we have not yet instanciated any
session at all. Let's use a new counter for that.
diff --git a/include/types/counters.h b/include/types/counters.h
index 7a0ff1d..a333219 100644
--- a/include/types/counters.h
+++ b/include/types/counters.h
@@ -40,6 +40,7 @@
 
 	long long denied_req, denied_resp;	/* blocked requests/responses because of security concerns */
 	long long failed_req;			/* failed requests (eg: invalid or timeout) */
+	long long denied_conn;			/* denied connection requests (tcp-req rules) */
 
 	union {
 		struct {
@@ -63,6 +64,7 @@
 
 	long long denied_req, denied_resp;	/* blocked requests/responses because of security concerns */
 	long long failed_req;			/* failed requests (eg: invalid or timeout) */
+	long long denied_conn;			/* denied connection requests (tcp-req rules) */
 };
 
 struct srvcounters {