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.
* Add options to Put, PutUnchecked and Update for all Fses
* Use these to create HashOption
* Implement this in local
* Pass the option in fs.Copy
This has the effect that we only calculate hashes we need to in the
local Fs which speeds up transfers significantly.
