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/client.h

@@ -47,7 +47,9 @@ struct client {
 	/* Have we seen a REQUEST_DISCONNECT message? */
 	int     disconnect;
 
-	/* kill the whole server if a request has been outstanding too long */
+	/* kill the whole server if a request has been outstanding too long,
+	 * assuming use_killswitch is set in serve
+	 */
 	timer_t killswitch;
 
 };