OSPFv3: Update neighbor authentication state from Hello packets

In OSPFv3, only Hello and DBDes packets contain flags specifying whether
RFC 7166 authentication trailer is used. Other packets are processed
based on stored authentication state in neighbor structure. Update this
state with each received Hello to handle authentication change from
reconfigurations.

Thanks to Joakim Tjernlund and Kenth Eriksson for the bugreport.