Make IP6-using clients try harder to find an IPv6 directory server

fascist_firewall_use_ipv6 doesn't take IPv6 ORPort preferences into account. (IPv6 DirPorts are deprecated, see legacy/trac#19704 (moved).)

This makes it skip IPv6 address checks, when it really should be trying harder to find them.

Part of legacy/trac#20996 (moved), opening this for the bug number.

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