This is a problem when syncing a file which just needed its modtime
set with dropbox which can't set the mod time of a file without
re-uploading it.
Before this change we would delete the file, then the server side move
would fail moving the file to the backup-dir because it no longer
existed.
After this change the destination file is moved to the backup-dir
instead of being deleted and the new file is uploaded.
Fixes#2134
A Range request can never request 0 bytes however this change was made
to make a clearer signal that the limit means read to the end.
Add test and more documentation and fixup uses
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
