[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/src/proto_tcp.c b/src/proto_tcp.c
index 3a1abad..1c93396 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -731,9 +731,9 @@
 		if (ret) {
 			/* we have a matching rule. */
 			if (rule->action == TCP_ACT_REJECT) {
-				s->fe->counters.denied_req++;
+				s->fe->counters.denied_conn++;
 				if (s->listener->counters)
-					s->listener->counters->denied_req++;
+					s->listener->counters->denied_conn++;
 
 				if (!(s->flags & SN_ERR_MASK))
 					s->flags |= SN_ERR_PRXCOND;