Respect net.add_addr_allfibs for IPv6
This patch contains two related changes:
First, we add routes for interface addresses to only the interface's FIB if net.add_addr_allfibs is 0. This is congruent with IPv4 behavior. This includes making changes to source address selection logic to ensure a source from the correct FIB is chosen. Previously, we could assume all interface routes were in the default FIB since interface addresses were added to all FIBs.
Second, default router selection (from router announcements) is now able to select multiple routers, one per FIB. Obviously, the selected router is installed as the default route in the FIB of the interface that received the RA.
See PR196361 - https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=196361