I'm able to get this thing to respond to duplicate packets.  I'll resume
ripping out libnet and see if that sticks around, but I don't like it.

Things I tried:

- Seeing if there were rogue sending processes that weren't killed by the
  interrupt (there didn't seem to be, either by ps or tcpdump)
- Adding and removing options (didn't seem to matter)

What to try next:

- tcpdump while doing a tcpping.  That should sort all of this out.

Hacks:

- Throw out any sequence number that we don't remember giving

** UPDATE **

After re-running, I can't manage to get this to do it again.  Not sure what's
up.  This obviously bothers me, but if I can't duplicate, there's not much I
can do.  Let me know if you encounter this!

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There's still one warning on Mac machines, but it's perfectly harmless AFAICT.

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I should try to drop privileges to the one needed for Linux and/or Mac OS X in
both the client and server.  It's a pretty limited set, after all.
