test: add truncate-and-rewrite one file
test: temporarily disable parallel truncate
undup_truncate: correctly call stub_close
undup_release: add tracing debug()
die: use exit rather than SEGV to terminate
stub_write: fix typo in debug message
stub_refresh: improve debug output
undup-core: improve debugging logging throughout

In particular, log fd in various places.
undup: invalidate fd and fh after close

chasing a thread race here; this commit doesn't fix the race
but it does close some obvious leakage opportunities and is
generally a good idea.
undup_create: close the correct fd (rather than 16) after creating the stub.