Create subscribers on demand

Add a new vty option and allow to optionally generate a random msisdn,
as well as setting the default NAM:

subscriber-create-on-demand (no-msisdn|<3-15>) (none|cs|ps|both)

Thanks to Vadim for the random MSISDN patch [1], which was squashed into
this one.

[1] Change-Id: I475c71f9902950fa7498855a616e1ec231fad6ac

Depends on: Idc74f4d94ad44b9fc1b6d43178f5f33d551ebfb1 (libosmocore)
Change-Id: I0c9fe93f5c24b5e9fefb513c4d049fb7ebd47ecd
Related: OS#2542
diff --git a/src/hlr.h b/src/hlr.h
index e2e96a4..18c4a1d 100644
--- a/src/hlr.h
+++ b/src/hlr.h
@@ -56,6 +56,11 @@
 	struct llist_head ss_sessions;
 
 	bool store_imei;
+
+	bool subscr_create_on_demand;
+	/* Bitmask of DB_SUBSCR_FLAG_* */
+	uint8_t subscr_create_on_demand_flags;
+	unsigned int subscr_create_on_demand_rand_msisdn_len;
 };
 
 extern struct hlr *g_hlr;