Add control command to expire subscriber

It's equivalent of existing vty command: common part is extracted into
shared helper function.

Change-Id: I267886b7c79ed6d9c2f34a2e60d2972b7f4f4036
diff --git a/include/osmocom/msc/vlr.h b/include/osmocom/msc/vlr.h
index b4bb27f..0b61a59 100644
--- a/include/osmocom/msc/vlr.h
+++ b/include/osmocom/msc/vlr.h
@@ -279,7 +279,7 @@
 
 /* tell the VLR that the subscriber connection is gone */
 int vlr_subscr_disconnected(struct vlr_subscr *vsub);
-
+bool vlr_subscr_expire(struct vlr_subscr *vsub);
 int vlr_subscr_rx_id_resp(struct vlr_subscr *vsub, const uint8_t *mi, size_t mi_len);
 int vlr_subscr_rx_auth_resp(struct vlr_subscr *vsub, bool is_r99, bool is_utran,
 			    const uint8_t *res, uint8_t res_len);