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

The reachable state might also be considered to keep the reachable state. This reduces the nbr discovery messages if nodes talk to each other periodically. Additionally, a comment has been added as well to document that this is not (and the previous code was neither) standard complaint behaviour but useful in many cases.
"and and" => "and"
"nbr" => "neighbor"
"ackes" => "acknowledges"