arp-v2 (void *)-1 "hack"

[Kip Macy]

> What do you think wrt to adding the (possibly optional) int *error and
> returning the errno rather than a (void *)-1?  If you'd be ok, I'd can
> prepare the patch. I'd rather break the API now than in a few months.

I would greatly prefer having a dedicated new function that calls in
to it. There are a lot of calls to lla_lookup that would have to be
changed needlessly. In other words:
1) lla_lookup_internal - a static function which takes all 3 args
2) lla_delete - which returns an errno
3) lla_lookup - which maintains the current interface

I'll review the patch as soon as it is ready.

-Kip