IPv6 Support

[BenjaminEHowe]

It looks like network-manager-l2tp doesn't support IPv6. Please could support be considered?

[dkosovic]

xl2tpd doesn't support IPv6, so I've never really considered adding IPv6 support.

The xl2tpd ipv6 branch in GitHub hasn't been touched in 7 years and I'm not sure if it will even build or work with modern linux kernels:

• https://github.com/xelerance/xl2tpd/tree/ipv6