In some cases addresses come from an external source as a string or as a
`struct sockaddr_in6`, without a tag to tell whether it is a private
IPv6 or a CJDNS address. In those cases interpret the address as a CJDNS
address instead of an IPv6 address if `-cjdnsreachable` is set and the
seemingly-IPv6-address belongs to `fc00::/8`. Those external sources are:
* `-externalip=`
* `-bind=`
* UPnP
* `getifaddrs(3)` (called through `-discover`)
* `addnode`
* `connect`
* incoming connections (returned by `accept(2)`)
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