Add ClientPreferIPv6DirPort to express a preference for IPv6 directory connections over IPv4 directory connections. Tor already has ClientPreferIPv6ORPort, which expresses a preference for IPv6 OR connections over IPv4 OR connections. (ClientUseIPv6 allows the use of IPv6 connections, ClientPreferIPv6ORPort/ClientPreferIPv6DirPort are the tie-breakers if IPv4 and IPv6 are both equally likely.) While we're doing this, let's warn on nonsensical situations like ClientPreferIPv6* 1 ClientUseIPv6 0. (Although ClientPreferIPv6* 0 ClientUseIPv6 1 should be an info or notice, as ClientPreferIPv6* 0 is the default.)