Default log file is handled inconsistently

Our default logging setting is either "Log notice stdout" or "Log warn stdout" or "", depending on the command-line options. Right now, this is set in options_validate, which should not be setting anything at all. It's also set at the wrong time: defaults should be set in get_options_defaults(), so that they have the right semantics when you try to replace them or extend them.

I'll be fixing this as part of the parent ticket, but I'm making a separate entry here so we can refer to this bug in particular.