IPv6 SLAAC: Clamp received PIO and RIO Lifetime Values
This commit implements Section 4.1.2 of https://tools.ietf.org/html/draft-gont-6man-slaac-renum-05, to improve the reaction of IPv6 SLAAC to renumbering events.
Namely:
- It caps the Preferred Lifetime of PIOs to the "Router Lifetime" value of the corresponding Router Advertisement Message, and the Valid Lifetime of PIOs to 48 * Router Lifetime.
Additionally, it also caps the Valid Lifetime of RIOs to "Router Lifetime".