MINOR: counters: make it easier to extend the amount of tracked counters

By properly affecting the flags and values, it becomes easier to add
more tracked counters, for example for experimentation. It also slightly
reduces the code and the number of tests. No counters were added with
this patch.
diff --git a/include/types/proto_tcp.h b/include/types/proto_tcp.h
index 0ee2ec7..74863b4 100644
--- a/include/types/proto_tcp.h
+++ b/include/types/proto_tcp.h
@@ -32,7 +32,7 @@
 enum {
 	TCP_ACT_ACCEPT = 1,
 	TCP_ACT_REJECT = 2,
-	TCP_ACT_TRK_SC1 = 3,
+	TCP_ACT_TRK_SC1 = 3, /* TCP request tracking : must be contiguous */
 	TCP_ACT_TRK_SC2 = 4,
 };