ipv6: added address,prefix,nduseropt event tracers

Moved debug dumps from event processing code to explicit
trace functions usable as event callback handlers.
wickedd: enabled address,prefix,nduseropt events
dhcp6: try to start on device-change event in auto mode
address: expose flags via dbus and show in ifstatus
Merge pull request #399 from mtomaschewski/bnc889981