Before this change rclone didn't use sparse files on Windows. This
means that when you downloaded a file with multithread download it
wrote the entire file with zeros first on the first write not at the
start of the file.
This change makes the file be sparse on Windows. Linux/macOS files
were already sparse.
Under Linux, rclone attempts to preallocate files for efficiency.
Before this change, pre-allocation would fail on ZFS with the error
Failed to pre-allocate: operation not supported
After this change rclone tries a different flag combination for ZFS
then disables pre-allocate if that doesn't work.
Fixes#3066
