IPv6 SLAAC: Honor small PIO Valid Lifetimes
This commit implements Section 4.2 of https://tools.ietf.org/html/draft-gont-6man-slaac-renum-05, to improve the reaction of IPv6 SLAAC to renumbering events.
Namely:
- It honors PIO Valid Lifetimes < 2 hours, deviating from item "e)" (pp. 19-20) of Section 5.5.3 of RFC4862.