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/src/libmsc/vty_interface_layer3.c b/src/libmsc/vty_interface_layer3.c
index 65ba178..c7ec586 100644
--- a/src/libmsc/vty_interface_layer3.c
+++ b/src/libmsc/vty_interface_layer3.c
@@ -572,12 +572,9 @@
 		return CMD_WARNING;
 	}
 
-	if (vsub->lu_complete) {
-		vsub->lu_complete = false;
-		vlr_subscr_put(vsub);
+	if (vlr_subscr_expire(vsub))
 		vty_out(vty, "%% VLR released subscriber %s%s",
 			vlr_subscr_name(vsub), VTY_NEWLINE);
-	}
 
 	if (vsub->use_count > 1)
 		vty_out(vty, "%% Subscriber %s is still in use,"