[This was broken when the 4.4BSD ARP table changes were done, and never
repaired.]
Inspired by Jarle Greipsland, PR 4531; code stolen from arp.a and cleaned
up (mostly removing global variables).
XXX As pointed out in the PR, this should be in some library (libutil?),
to be usable by other servers like bootpd and dhcpd.
is