Add VTY command to configure destination MSISDN for emergency calls

As in GSM/3GPP networks emergency calls carry no explicit destination
number/address, add a VTY commadn to patch in some destination handler
in the EMERGENCY SETUP before delivering to [internal or external] MNCC.

Change-Id: I7c9f43ba312fadda2b9a9483b3cf50e4abca9599
diff --git a/include/osmocom/msc/gsm_data.h b/include/osmocom/msc/gsm_data.h
index 3f322b3..06328dc 100644
--- a/include/osmocom/msc/gsm_data.h
+++ b/include/osmocom/msc/gsm_data.h
@@ -413,6 +413,11 @@
 		struct llist_head bscs;
 		struct osmo_sccp_instance *sccp;
 	} a;
+
+	struct {
+		/* MSISDN to which to route MO emergency calls */
+		char *route_to_msisdn;
+	} emergency;
 };
 
 struct osmo_esme;