NDP EUI64 lookups for IPv6 clients

[Opendium]

Adds support for looking up an IPv6 client's MAC address from the neighbour cache. The resulting MAC address is converted to an EUI64 address. Previously, the EUI64 was only available for IPv6 clients with site-local SLAAC addresses, which almost never occur in the wild.

Communication with the kernel is done through a Netlink socket, which adds blocking I/O to many Squid transactions.

[squid-prbot]

Can one of the admins verify this patch?

[rousskov]

I did not review the code changes. They could use some style polishing, but I hope others will take care of that.

@Opendium, if this does not happen in a week or two, please request my review, and I will help.