Add a missing ! to directory_fetches_from_authorities

Add a missing ! to directory_fetches_from_authorities

The old logic would have us fetch from authorities if we were refusing
unknown exits and our exit policy was reject*.  Instead, we want to
fetch from authorities if we're refusing unknown exits and our exit
policy is _NOT_ reject*.

Fixed by boboper.  Fixes more of 2097.  Bugfix on 0.2.2.16-alpha.