Make extending subscriber creation easier

* rename variable controlling subscriber creation
* use enum for subscriber creation policy
* move check for subscriber creation policy into separate static
  function

Related: OS#1658, OS#1647
Change-Id: I3b10a9a764fd3a7bb96717a990e52caae16266da
Reviewed-on: https://gerrit.osmocom.org/42
Tested-by: Jenkins Builder
Reviewed-by: Holger Freyther <holger@freyther.de>
diff --git a/openbsc/src/libbsc/net_init.c b/openbsc/src/libbsc/net_init.c
index 568a0b8..afcaaf3 100644
--- a/openbsc/src/libbsc/net_init.c
+++ b/openbsc/src/libbsc/net_init.c
@@ -48,7 +48,7 @@
 	INIT_LLIST_HEAD(&net->bsc_data->mscs);
 
 	net->subscr_group->net = net;
-	net->create_subscriber = 1;
+	net->subscr_creation_mode = GSM_SUBSCR_CREAT_W_RAND_EXT;
 
 	net->country_code = country_code;
 	net->network_code = network_code;