sms: Add a way to always route SMS through SMPP systems

default-route would only be looked at after there has been
no subscriber in the local database. Depending on the setup
this is not what one wants. This has been discussed at the
OsmoDevCon and there have been hacks in some branches. Let's
introduce a VTY command to select if SMPP should be consulted
first and then fallback to the current behavior.
diff --git a/openbsc/src/libmsc/smpp_openbsc.c b/openbsc/src/libmsc/smpp_openbsc.c
index b17222f..057a9d0 100644
--- a/openbsc/src/libmsc/smpp_openbsc.c
+++ b/openbsc/src/libmsc/smpp_openbsc.c
@@ -539,6 +539,11 @@
 
 static struct smsc *g_smsc;
 
+int smpp_route_smpp_first(struct gsm_sms *sms, struct gsm_subscriber_connection *conn)
+{
+	return g_smsc->smpp_first;
+}
+
 int smpp_try_deliver(struct gsm_sms *sms, struct gsm_subscriber_connection *conn)
 {
 	struct osmo_esme *esme;