gsup/oap: add OAP to GSUP client.

Trigger an OAP registration upon IPA connect. Feed incoming OAP messages to
oap_handle() and send replies returned by it.

Add oap_config to sgsn_config (todo: vty).

Sponsored-by: On-Waves ehf

[hfreyther: Fix coding style]
diff --git a/openbsc/src/gprs/gprs_subscriber.c b/openbsc/src/gprs/gprs_subscriber.c
index 8231e8c..3467293 100644
--- a/openbsc/src/gprs/gprs_subscriber.c
+++ b/openbsc/src/gprs/gprs_subscriber.c
@@ -63,7 +63,8 @@
 
 	sgi->gsup_client = gprs_gsup_client_create(
 		addr_str, sgi->cfg.gsup_server_port,
-		&gsup_read_cb);
+		&gsup_read_cb,
+		&sgi->cfg.oap);
 
 	if (!sgi->gsup_client)
 		return -1;