tor-config: default fs-mistrust should tolerate readable config files

Right now, if you don't call set_mistrust on your ConfigurationSources, you get a default Mistrust object.

That's probably not right default for configuration files: The default mistrust object forbids configuration files that are readable by untrusted users, when previously we only forbade those that were _writable` by untrusted users.

I can make this change, but first I want to make sure that the behavioral change wasn't intentional. cc @Diziet