Figure out own IPv6 address

A relay should be able to figure out if the system has got an IPv6 address configured, just like what's done for IPv4.

Should we turn resolve_my_address() into resolve_my_addresses() and teach it about IPv6? get_interface_address6() used here needs some work for #4806 (moved) too.

A few thoughts:

resolve_my_address() looks at options->Address. What should 'Address' mean now that a relay doesn't have one single address any more?
get_interface_address() says "This address should only be used in checking whether our address has changed" but is actually used by resolve_my_address() in the case where we fail to resolve our hostname. Does get_interface_address6() need more work or should we just add a comment to where we use it in a non-recommended way?

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