Fix IPv6 DNS server adddress selection

Fix selection of IPv6 DNS address in DHCPv6 and RA overwrite as an address could be selected
with preferred lifetime zero while IPv6 addresses are in use with non zero preferred
lifetimes.
Fix tries to pick the IPv6 address with the longest preferred lifetime now.
Fixes also the issue an IPv6 address with preferred lifetime zero could be returned in DHCPv6
DNS server option while an IPv6 address with non zero preferred lifetime is returned as DNS
recursive RA option for the same set of available IPv6 addresses.
Merge pull request #30 from dedeckeh/bugfixes

Fix IPv6 DNS server adddress selection