Fix IPv6 bridges with a private/dynamic IPv4 address

A bridge not listening to an IPv4 address looks OK (bootstraps 100%) but leaves the client hanging at 50%.

Workaround: Specify an ORPort on an IPv4 address in addition to the IPv6 address.

An idea is that it's related to the self testing that the bridge is supposed to perform. (Setting AssumeReachable 1 hasn't changed anything in my tests though.)