Warn when IPv6Exits have no ipv6-policy line in their descriptor
There appears to be a bug in Tor's IPv6 Exit code, where exits that have IPv6Exit set do not have an ipv6-policy line in their descriptor.
To assist in diagnosing this bug, we should log a warning message (not a bug message) containing the full exit policy, when the IPv6 exit policy summary is empty, but IPv6Exit is 1 and ExitRelay is 1 or auto.