tor-config: Provide resolve_option_general, for T: !Default etc.
At one point in !602 (merged) I thought I was going to want this for arti::cfg::ListenConfig (which we don't want to be Default).
In fact ListenConfig is being handled specially, but having written this function it seemed sensible to keep it. Since resolve_option becomes a wrapper for it, the existing tests exercise it.
(Assigning @nickm as reviewer as per comments in !602 (merged).)