nd6: Add support for route information (RFC 4191)

Implement RFC 4191 by handling received Router Adverisement (RA)
packets with route information option.
For default routes, use the route information's lifetime and
preference to overwrite the RA's lifetime/preference.
Also install and update more-specific route prefixes with the
option's lifetime and expire them when their lifetime elapses.

PR: 263982
Reviewed by: markj
Tested by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D55449