nat: Use libctrl and add command forwarding to osmo-bsc

Passes commands beginning with "bsc.<num>" to the bsc that is
responsible for LAC <num>.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c b/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
index 4834340..8658c3d 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
@@ -129,6 +129,7 @@
 
 	con->nat = nat;
 	osmo_wqueue_init(&con->write_queue, 100);
+	INIT_LLIST_HEAD(&con->cmd_pending);
 	return con;
 }