ip6addrctl_enable is set to YES, address selection policy is installed
into kernel.
If there is /etc/ip6addrctl.conf, it is used for address selection
policy. Even if there is no /etc/ip6addrctl.conf, we install default
policy. In this case, if ipv6_enable is set to YES, we use address
selection policy described in RFC 3484 as default. Otherwise, we
install priority policy for IPv4 address.
The default of ip6addrctl_enable is NO for now. However, it may
better to enable it by default.