Series of IPv6 Improvements

Hi,

I've recently completed a series of changes focused on enhancing our 
IPv6 stack, particularly around several important RFCs related to 
Neighbor Discovery Protocol (NDP). These changes not only resolve 
long-standing issues but also introduce new features that align with 
modern networking standards.

Here's an overview of what I've done:
- Implemented Gratuitous Neighbor Discovery (RFC 9131 + RFC 4861 Section 
7.2.6, which is also addresses one of the IPv6 deployment issues in RFC 
9898 Section 3.9.)
- Implemented Optimistic DAD (RFC 4429)
- Added support for delayed anycast and proxy addresses during Neighbor 
Advertisement (RFC4861 7.2.7, + 7.2.8)
- Introduced support for multiple PREF64 in Router Advertisement (RFC 8781)
- Implemented support for Route Information option (RFC 4191)
- Simplified `nd6_ra_input()`

While implementing the RFC 4191, I found out some of the route 
expiration facilities are broken. So I fixed it myself:
- rtnetlink(4): Add support for nexthop expiration in new route
- route(8): Fixed the -expire argument when using netlink
- netstat(1): Fixed expire column values in -r flag when using netlink
- snl(3): Remove duplicate `rta_expires` member in the route parser

These improvements should enhance our IPv6 capabilities significantly 
and resolves many /* XXX */ comments + some long-standing issues like 
lack of route option support (see PR263982).

You can find those reviews here:
https://reviews.freebsd.org/search/query/FpCNDQh8WgIb

I'd appreciate any review or testing these proposed changes on Phabricator.

-- 
Pouria

-----BEGIN PGP SIGNATURE-----

iHUEARYKAB0WIQSqt7cppfvJ816gj0lUwVnUeMwagAUCaZwaAAAKCRBUwVnUeMwa
gFDmAQCZ+hS4Ce8ewTk6zGyiS3Gz6mTFWKsdklx/pXgD8+sEUwEA9clovmz+Hc9z
oa7RpT9iTnsv0BcnEkpABUn35h4cmQs=
=9UEW
-----END PGP SIGNATURE-----