gprs_ns2_sns: rework tracking of NS-VC unblocked/alive state

The SNS must know when all NS-VC have failed. Further more
there might be a corner case when the SNS configuration succeeds but
no NS-VC comes up afterwards.

Related: OS#5355
Change-Id: Ie72da9adeefe0c2850d49a9208b2d0a4556f9101
diff --git a/src/gb/gprs_ns2_internal.h b/src/gb/gprs_ns2_internal.h
index 6462907..ff95c81 100644
--- a/src/gb/gprs_ns2_internal.h
+++ b/src/gb/gprs_ns2_internal.h
@@ -350,7 +350,8 @@
 int ns2_sns_rx(struct gprs_ns2_vc *nsvc, struct msgb *msg, struct tlv_parsed *tp);
 struct osmo_fsm_inst *ns2_sns_bss_fsm_alloc(struct gprs_ns2_nse *nse,
 					     const char *id);
-void ns2_sns_free_nsvc(struct gprs_ns2_vc *nsvc);
+void ns2_sns_replace_nsvc(struct gprs_ns2_vc *nsvc);
+void ns2_sns_notify_alive(struct gprs_ns2_nse *nse, struct gprs_ns2_vc *nsvc, bool alive);
 
 /* vc */
 struct osmo_fsm_inst *ns2_vc_fsm_alloc(struct gprs_ns2_vc *nsvc,