A regular file replaced by a FIFO after the walk statted it made the
hash phase's open wait forever for a writer, so the scan never ended
and kept the lock. The hash and content phases now open each file with
O_NONBLOCK and check with fstat that it is still a regular file;
anything else is a per-path error, warned about and skipped. The new
test replaces a walked file with a FIFO before the hash phase reads it.
Model: opus-5-5
sneak