Needed to maintain {mac:ip} assocation on the switch port. This likely
fixes many of the reported pkt-gen issues.

Also added PHY link state poll ("auto wait") when '-w0' is specified.

The check is now only done on thread 0, and before touching the
slot buffer we check if slot->len == sizeof(ARP packet), ie 42,
whereas the smallest packet size pkt-gen does without crashing
is 50. (This commit also includes a better minimum packet size
constraint for the -l argument).

Some related code has been tidied up, and I've added #defines
for IPv4 and MAC address sizes (IP4_ALEN, ETH_ALEN) to get rid
of some magic numbers.