ggsn: Add per-APN VTY configuration option on G-PDU sequence numbers

This per-APN vty option determines if we are transmitting GTP sequence
numbers in downlink G-PDU messages.  This behavior is optional as per
GTP spec.  The default behavior is "true", like before this change.

Related: OS#2519
Change-Id: Ibf0de261f83951309b01b4feae998b6656c77664
diff --git a/ggsn/ggsn.c b/ggsn/ggsn.c
index d521e57..780a0c2 100644
--- a/ggsn/ggsn.c
+++ b/ggsn/ggsn.c
@@ -517,6 +517,9 @@
 
 	process_pco(apn, pdp);
 
+	/* Transmit G-PDU sequence numbers (only) if configured in APN */
+	pdp->tx_gpdu_seq = apn->cfg.tx_gpdu_seq;
+
 	LOGPPDP(LOGL_INFO, pdp, "Successful PDP Context Creation: APN=%s(%s), TEIC=%u, IP=%s\n",
 		name_buf, apn->cfg.name, pdp->teic_own, in46a_ntoa(&member->addr));
 	gtp_create_context_resp(gsn, pdp, GTPCAUSE_ACC_REQ);