write_to_pkts():
If for some weird reason a downlink's address is in the PATH, but not in the
SEEN-BYs, detect as a loop and don't send to (add to an outbound pkt for) the
downlink.

This is an experimental change to see if it addresses the issue reported by
Richard Williamson and Mark Lewis with regards to dupes in the COOKING
echo from point nodes off 1:396/45.