Finding gifts for a dad who wants nothing can be surprisingly difficult. If he already buys the things he needs, says he doesn't want any more stuff or is simply hard to shop for, another novelty mug or gadget destined for a drawer probably isn't the answer.For Father's Day, I've focused on useful tech that earns its place: things that solve an everyday annoyance, upgrade something Dad already uses or make one of his hobbies a little better. Tha…