Handle buggy prefixes (link local fe80:...) handed out by DHCPv6

One of the differences between the previous and the current IPv6 code is that the previous version was looking at the router advertisements to decide if IA_NA and/or DNS should be requested.
Current version always tries requesting IA_NA (with a fallback inside odhcp6c).

Unfortunately, at least one ISP will consistently hand out bogus prefixes when both IA_NA and IA_PD are requested (even though no address will be returned and only one prefix starting with fe80: is returned).
One way to work around this is by passing "-N none" to odhcp6c, even though this should not be necessary. But for some reason the fallback in odhcp6c does not seem to work.