A WR socket registers using a specific mac address and the receiving code
uses the mac address to match the frame.  This is going to be a problem
with the new PTP code, as pdelay frames use a different mcast mac address.
Moreover, if we go to UDP, stuff gets worse.

Packet filter rules are already in place for the new mac addresses, and
we now need to relax the checking in the software stack.

As a side effect, we avoid a number of broadcast frames, because wrpc-sw
now replies to unicast ARP requests.

What happened before:

   16:27:07 00:0b:ab:7c:8e:4a > 00:26:7b:00:04:18 who-has ...107 tell ...200
   16:27:08 00:0b:ab:7c:8e:4a > 00:26:7b:00:04:18 who-has ...107 tell ...200
   16:27:09 00:0b:ab:7c:8e:4a > 00:26:7b:00:04:18 who-has ...107 tell ...200
   16:27:12 00:0b:ab:7c:8e:4a > ff:ff:ff:ff:ff:ff who-has ...107 tell ...200
   16:27:12 00:26:7b:00:04:18 > 00:0b:ab:7c:8e:4a ...107 is-at 00:26:7b:00:04:18

what happens now:

   16:42:27 00:0b:ab:7c:8e:4a > 00:26:7b:00:04:18 who-has ...107 tell ...200
   16:42:27 00:26:7b:00:04:18 > 00:0b:ab:7c:8e:4a ...107 is-at 00:26:7b:00:04:18
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>