Make ClientPreferIPv6ORPort apply to bridge clients

After legacy/trac#17840 (moved), clients that don't use bridges will select ORPorts using ClientPreferIPv6ORPort.

We could do this for bridges as well, by modifying the new function fascist_firewall_choose_address_base:

use ClientPreferIPv6ORPort to choose a preferred address for bridge clients,
but ignore the "preferred address only setting", so that bridge users always get an address if there is one available.

I don't want to do this as part of legacy/trac#17840 (moved), because I'm not sure if it's really necessary - bridges just seem to work ok as-is.

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