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

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.

Reviewed by: glebius
Differential revision: https://reviews.freebsd.org/D35536