uip-ds6-nbr: consider reachable node if LL-NUD

The reachable state might also be considered to keep the reachable
state since we are already not standard complaint. This reduces the
nbr discovery messages if nodes talk to each other periodically.
Merge pull request #836 from lebrush/nbr-ll-nup

uip-ds6-nbr: consider reachable node if LL-NUD