arp(8): allow -i with -s

[llfw]

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.

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cc @glebius

this PR came about from my attempts to make proxy ARP work in FreeBSD -- i still couldn't get it to work for my purposes even with this PR, but i think this is useful even so.

[glebius]

@AlexanderChernikov can you please take a look, too?

[llfw]

hi @bsdimp what changes are required here?

[gmshake]

hi @bsdimp what changes are required here?

I think that tag should be need-review :)

[bsdimp]

yea, waiting for @glebius or @AlexanderChernikov (or maybe @gmshake) to review it.