bridge: output complete bridge lines to logs & disk
Solves #29128.
Currently, this patch copies pt_get_extra_info_descriptor_string
's behavior of ignoring the bridge's IPv6 adresses if it has an IPv4.
IMO it'd be better to make get_extra_info
(and this patch) output both addresses when possible - it would effectively give us many more IPv6 bridges basically for free. That's tangential to this issue, though.
(I've also included a minor bugfix for an incorrect AssumeReachableIPv6 check. Hopefully that's fine to include in this MR?)