Automatically adjust adrfam when address is not IPv4 when creating listener

I set an IPv6 address for the gateway and followed the examples in the README. The add listener step failed because the adrfam of the gateway was left as "ipv4". You can add `-f ipv6" as an extra option and then it works. But it would be nice to have the gateway fix this for me behind te covers

Start with the README listener-creation examples and trace the gateway code used by the add listener step. Reproduce the failure with an IPv6 gateway while omitting the explicit -f ipv6 option, then verify that listener creation derives the correct adrfam automatically and still works for IPv4.

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