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.
Before this change on Windows, files copied locally could become
heavily fragmented (300+ fragments for maybe 100 MB), no matter how
much contiguous free space there was (even if it's over 1TiB). This
can needlessly yet severely adversely affect performance on hard
disks.
This changes uses NtSetInformationFile to pre-allocate the space to
avoid this.
It does nothing on other OSes other than Windows.
