flexnbd: Make the killswitch runtime-selectable

We're not actually using it in production right now because it doesn't
shut its sockets down cleanly enough. This is a better option than
reverting the functionality or keeping production downgraded until
we sort out a handler that cleanly closes the sockets.

src/flexnbd.h

@@ -36,7 +36,8 @@ struct flexnbd * flexnbd_create_serving(
 	int default_deny,
 	int acl_entries,
 	char** s_acl_entries,
-	int max_nbd_clients);
+	int max_nbd_clients,
+	int use_killswitch);
 
 struct flexnbd * flexnbd_create_listening(
 	char* s_ip_address,