unify allocation of gsm_subscriber_connection

The current msc_subscr_con_allocate() was in fact only used by msc_vlr_tests,
while both a_iface_bssap.c and iucs.c did their own duplicate code of
allocating the gsm_subscriber_connection struct. Unify.

Drop the old msc_subscr_con_allocate(), instead add msc_subscr_conn_alloc().
The new function also takes via_ran and lac arguments directly.

The conn allocation will soon be closely tied to the subscr_conn_fsm instance
allocation, so place the new function definition alongside the other
subscr_conn_fsm API, and match its naming ("conn").

Related: OS#3122
Change-Id: Ia57b42a149a43f9c370b1310e2e1f512183993ea
diff --git a/tests/msc_vlr/msc_vlr_test_rest.c b/tests/msc_vlr/msc_vlr_test_rest.c
index 3f843f0..7fd7bae 100644
--- a/tests/msc_vlr/msc_vlr_test_rest.c
+++ b/tests/msc_vlr/msc_vlr_test_rest.c
@@ -31,7 +31,7 @@
 	EXPECT_ACCEPTED(false);
 
 	btw("freshly allocated conn");
-	g_conn = msc_subscr_con_allocate(net);
+	g_conn = msc_subscr_conn_alloc(net, RAN_GERAN_A, 123);
 	EXPECT_ACCEPTED(false);
 
 	btw("conn_fsm present, in state NEW");