WIP: handle NDP router sollicitations

NDP RS should allow to dynamically assign IPv6 address to the guest. It is similar to DHCP but for IPv6.

To be on par with libslirp, I have started adding NDP Router Solicitication support.

It should be farily small addition to slirpnetstack, most of the work is on netstack itself: https://github.com/google/gvisor/pull/1810