Merge pull request #338 from mtomaschewski/dhcp-release

dhcp6: do not break ifdown if release fails (bnc#884012)
ifdown: flush addrs and routes on device-down (bnc#885236)
Merge pull request #339 from mtomaschewski/ifdown-flush

ifdown: flush addrs and routes on device-down (bnc#885236)