Fix receive filter handling. Update the unicast mac address in the filter

Fix receive filter handling.  Update the unicast mac address in the filter
when bringing the interface up, so it's possible to change it with
ifconfig.  Rearrange multicast handling so IFF_ALLMULTI is set correctly
and multicast ranges don't trigger promisc mode.  Perhaps most importantly,
set RPF_MCAST_FILTER_EN to 0 when we don't want all multicast traffic -
we use mac address filters to receive specific multicast groups, and the
multicast filter is programmed to accept all multicast traffic.

based on a diff from Brad
ok dlg@