nd6: centralize link-local packet send decision

Previously, ethip6 and lowpan6 each had their own copy of code that
used internal nd6 data structures to decide whether to send a packet
on the local link right away, or queue it while nd6 performed local
address resolution.  This patch moves that code into nd6, thereby
eliminating all remaining cases of external access to internal nd6
data structures, as well as the need to expose two specific nd6

As a side effect, the patch effectively fixes two bugs in the lowpan6
code that were already fixed in the ethip6 code.
4 files changed