Even more loop-prevention paranoia:

If a packed messages contains no PATH or SEEN-BY lines, we can still detect
and prevent a message loop by comparing the origin address in the packet header
against the downlink's address and if it's a match, skip that downlink.
It is still possible that a packed message header contains a different origin
address than the packet header, and we're actually over-writing the packed
messge header variable with the parsed Origin: line address (if there is one),
so perhaps we'll want to compare the (actual) packed message header origin
address too at some point in the future, if loops continue to be a problem.