Implement rule 5.5 of RFC 6724 (Default Address Selection for IPv6)

Implement rule 5.5 of RFC 6724 (Default Address Selection for IPv6)

Rule 5.5: Prefer addresses in a prefix advertised by the next-hop.

For this we have to track the (link-local) address of the advertising
router per interface address and compare it with the selected route.

Rule 5.5 is useful in multi-homing setups where we have more than one
prefix and default router. We have to use the source address with the
correct default gateway otherwise traffic is likely going to be
dropped because of BCP 38.

While here refactor in6_update_ifa() a bit to make the code clearer
and consistently use (var & flag) instead of (var & flag) != 0.

Patiently reviewed by & OK bluhm.