Code review

Code review

1. (error) Fix the --work-offline flag.
   It has been unused since commit
   85478ff30ebc029abb0ded48de5b557f52a758e0.
2. (performance) Remove redundant fopen() and fclose() during
   valid_file_or_dir().
   If stat() is used instead of fstat(), there's no need to open and
   close the file.
   (Technically, it's no longer validating readabilty, but since the
   validator downloads the files, read permission errors should be
   extremely rare, and can be catched later.)
3. (fine) Remove return value from thread_pool_task_cb.
   This wasn't a problem, but the return value was meaningless, and
   no callers were using it.