arp(8): use getifaddrs(3) instead of ioctl(SIOCGIFCONF) etc.

[takahiro.kurosawa_gmail.com]

Summary

The original code had used a fixed-size buffer for ioctl(SIOCGIFCONF),
that might cause the target ifreq spilled from the buffer.

Use the handy getifaddrs(3) to fix the problem.

Test Plan

Run the following script and check the output of ${arp} -s 198.51.100.200 auto pub

#!/bin/sh

naddrs=32
arp=/usr/sbin/arp

ifconfig epair0 create
ifconfig epair0a up
ifconfig epair0b up

i=1
while [ $i -le $naddrs ]
do
        ifconfig epair0a inet 198.51.100.${i}/25 alias
        i=$((i+1))
done

ifconfig epair0b ether
ifconfig epair0b inet 198.51.100.129/25
${arp} -s 198.51.100.200 auto pub
${arp} -n 198.51.100.200

ifconfig epair0a destroy

[glebius]

Thanks for nice submission!