dhcp6: fixed to accept low preference server offers

We wait until retransmit timeout before we accept
offers from low preference servers.
We may make it configurable how long to wait later.
Merge pull request #87 from mtomaschewski/dhcp-request-2

dhcp6: fixed to accept low preference server offers
Merge pull request #88 from olafhering/olh_ni_nl_talk_error_handling