arp(8): fix by-interface and by-host filtering when using netlink

arp(8) has traditionally supported filtering by interface via -i and
by hostname. However, this functionality was omitted from the initial
netlink-ification of arp. This patch re-introduces this filtering
functionality.

This patch also improves by-interface filtering by storing and using the
ifindex of the requested interface for filtering instead of comparing
interface name strings

Reviewed by: melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")