[nat] Add a token to the nat config and handle ID GET

This allows to chain a nat with a nat by answering to the
id get code and sending the token.
diff --git a/openbsc/include/openbsc/bsc_nat.h b/openbsc/include/openbsc/bsc_nat.h
index 455e46e..090eca8 100644
--- a/openbsc/include/openbsc/bsc_nat.h
+++ b/openbsc/include/openbsc/bsc_nat.h
@@ -219,6 +219,7 @@
 	int msc_port;
 	int first_contact;
 	struct bsc_msc_connection *msc_con;
+	char *token;
 
 	/* timeouts */
 	int auth_timeout;