Treat 'ExitRelay 0' as a reject-all policy

Oops! Most folks use 'ExitPolicy reject :' to flag their relay as being a non-exit, but turns out that tor has another option that does the same thing...

https://www.torproject.org/docs/tor-manual.html.en#ExitRelay

We should change the Controller's get_exit_policy() method to provide a reject-all policy when this is set. Caught thanks to Gary on tor-relays...

https://lists.torproject.org/pipermail/tor-relays/2018-March/014741.html