=== Series of IPv6 Improvements

Contact: Pouria Mousavizadeh Tehrani <pouria@FreeBSD.org>

For the past 6 months, I have worked on refreshing our IPv6 stack and implementing pending RFCs in our stack, and they are now available on CURRENT.

So far, I have added the support for:

* RFC 9131 and RFC 9898 Section 3.9: Support for Gratuitous Neighbor Discovery (GRAND).
* RFC 8781 (Section 5): Ability to have multiple PREF64 options.
* RFC 4861 (Section 7.2.6 - 7.2.8): Support for delayed Neighbor Advertisement for both anycast and proxy addresses.
* RFC 4191: Support for Route Information option in Router Advertisement.
* Removed the support for DRAFT_IETF_6MAN_IPV6ONLY_FLAG to replace it with RFC 8925.

Ongoing works:
* RFC 8925: IPv6-Only Preferred Option For DHCPv4
link:https://reviews.freebsd.org/D56637[(D56637)]
* RFC 4429: Add support for Optimistic DAD.
link:https://reviews.freebsd.org/D55229[(D55229)]

I want to specially thanks mailto:glebius@FreeBSD[Gleb Smirnoff] for his timeless guidance and help on these works.
