Do not trigger global repair when forwarding error is detected
Rank error packet should not be forwarded
Drop forwarding error packet and send back DAO to originating parent
Force NUD on default routers
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 #652 from cetic/pr-rpl-rank-and-fw-errors

RPL rank and forward errors
Merge pull request #836 from lebrush/nbr-ll-nup

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