dhcp6: do not break ifdown if release fails (bnc#884012)

When the interface is not ready to send out a lease release,
commit the lease drop and notify wickedd, so ifdown goes on.
ifdown: flush addrs and routes on device-down (bnc#885236)
Merge pull request #338 from mtomaschewski/dhcp-release
Merge pull request #339 from mtomaschewski/ifdown-flush