arpd replies to any ARP request for an IP address matching the specified
destination net with the hardware MAC address of the specified
interface, but only after determining if another host already claims it.

Any IP address cliamed by arpd is eventually forgotten after a period of
inactivity or after a hard timeout, and is relinquished if the real
owner shows up.

This enables a single host to claim all unassigned addresses on a LAN
for network monitoring or simulation.
