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

[rcm]

Summary

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

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

[kp]

Give melifaro@ a few days to review too though.

[melifaro]

usr.sbin/arp/arp.c

161

You should br able to pass ifname as a filter to the kernel netlink request directly

usr.sbin/arp/arp_netlink.c

313

It should be filtered in the kernel already

[rcm]

usr.sbin/arp/arp.c

161

cool, I admit I don't know much about netlink beyond what it took to work on this patch. I will look at that! thanks

[rcm]

Removed the unnecessary userspace filtering by-interface, the kernel is already doing
that now which I accidentally fixed by calling print_entries_nl with a valid (non-zero) ifindex

[kp]

Committed in https://cgit.freebsd.org/src/commit/?id=79278872ad966e5f54805efbeb692c8cbc0306c8 but I forgot to add the 'Differential Revision' tag, so it didn't auto-close.