[GPRS] NS: VTY: Move all local ip/port bind values into 'ns' node

This removes the requirement for gb_proxy and sgsn to have duplicate
vty parsing code
diff --git a/openbsc/src/gprs/sgsn_main.c b/openbsc/src/gprs/sgsn_main.c
index 63ef9f5..1956428 100644
--- a/openbsc/src/gprs/sgsn_main.c
+++ b/openbsc/src/gprs/sgsn_main.c
@@ -175,8 +175,9 @@
 	if (rc)
 		exit(2);
 
-	nsip_listen(sgsn_nsi, sgsn_inst.cfg.nsip_listen_ip,
-		    sgsn_inst.cfg.nsip_listen_port);
+	rc = gprs_ns_nsip_listen(sgsn_nsi);
+	if (rc)
+		exit(2);
 
 	while (1) {
 		rc = bsc_select_main(0);