sgsn: Integrate the GSUP client into the SGSN

This commit adds GSUP client configuration (via VTY), connection set
up, and real message sending.

The following configuration commands are added:

 - gsup remote-ip A.B.C.D            set server IP address
 - gsup remote-port PORT             set server TCP port

Ticket: OW#1338
Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/sgsn_main.c b/openbsc/src/gprs/sgsn_main.c
index 141eacf..f26b812 100644
--- a/openbsc/src/gprs/sgsn_main.c
+++ b/openbsc/src/gprs/sgsn_main.c
@@ -357,6 +357,12 @@
 		exit(2);
 	}
 
+	rc = gprs_subscr_init(&sgsn_inst);
+	if (rc < 0) {
+		LOGP(DGPRS, LOGL_FATAL, "Cannot set up subscriber management\n");
+		exit(2);
+	}
+
 	rc = gprs_ns_nsip_listen(sgsn_nsi);
 	if (rc < 0) {
 		LOGP(DGPRS, LOGL_FATAL, "Cannot bind/listen on NSIP socket\n");