Use get_interface6_via_udp_socket_hack() properly in _list().

When _list() is called with AF_UNSPEC family and fails to enumerate
network interfaces using platform specific API, have it call
_hack() twice to find out IPv4 and/or IPv6 address of a machine Tor
instance is running on. This is correct way to handle this case
because _hack() can only be called with AF_INET and AF_INET6 and
does not support any other address family.