arp(8): allow -i with -s

arp(8) usually disallows adding a static ARP entry for an IP address
which is not configured on a local interface.

Change this to allow such ARP entries to be added if '-i' is provided to
specify the interface the ARP entry relates to.

Due to limitations in the kernel lltable, this still requires that a
host route exists for the target address, but allows static ARP entries
to be configured to proxy ARP for, e.g., local jails which use an IPv4
address with a /32 route.

Reviewed by: imp, zlei
Pull Request: https://github.com/freebsd/freebsd-src/pull/1220