[MINOR] acl: add fe_conn, be_conn, queue, avg_queue

These ACLs are used to check the number of active connections on the
frontend, backend or in a backend's queue. The avg_queue returns the
average number of queued connections per server, and for this, divides
the total number of queued connections by the number of alive servers.

The dst_conn ACL has been slightly changed to more reflect its name and
original usage, which is to return the number of connections on the
destination address/port (the socket) and not the whole frontend.
diff --git a/src/client.c b/src/client.c
index 3bd31cf..b9e304a 100644
--- a/src/client.c
+++ b/src/client.c
@@ -593,12 +593,12 @@
 }
 
 
-/* set test->i to the number of connexions to the proxy */
+/* set test->i to the number of connexions to the same listening socket */
 static int
 acl_fetch_dconn(struct proxy *px, struct session *l4, void *l7, int dir,
                 struct acl_expr *expr, struct acl_test *test)
 {
-	test->i = px->feconn;
+	test->i = l4->listener->nbconn;
 	return 1;
 }