Finalization for a place channel used a recursive, non-atomic
function, which meant that a thread switch could happen during
place-channel finalization, leaving the new thread with the
master GC and generally confused. (The random-message test
found the bug right away on my machine.)
We already have a non-recursive, non-atomic function to traverse
place messages, so collapse all modes into that one implementation.
Along the way, problems with empty structs (found by random tester)
and checking of file descriptors (test added) also fixed.
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