handle it in arc_output() instead of nd6_storelladdr().
* Remove IFT_ARCNET check from arpresolve() since arc_output() does not
use arpresolve() to handle broadcast/multicast. This check was there
since r84931. It looks like it was not used since r89099 (initial
import of Arcnet support where multicast is handled separately).
* Remove IFT_IEEE1394 case from nd6_storelladdr() since firewire_output()
calles nd6_storelladdr() for unicast addresses only.
* Remove IFT_ARCNET case from nd6_storelladdr() since arc_output() now
handles multicast by itself.
As a result, we have the following pattern: all non-ethernet-style
media have their own multicast map handling inside their appropriate
routines. On the other hand, arpresolve() (and nd6_storelladdr()) which
meant to be 'generic' ones de-facto handles ethernet-only multicast maps.
MFC after: 3 weeks
d63e657c04