This simplifies the implementation of remotes. The only required
interface is now `List` which is a simple one level directory list.
Optionally remotes may implement `ListR` if they have an efficient way
of doing a recursive list.
If remote:path points to a file make NewFs return a sentinel error
fs.ErrorIsFile and an Fs which points to the parent.
Use this to remove the LimitedFs and just add this file to the
--files-from list.
This means that server side operations can be used also.
Fixes#518Fixes#545
This should fix duplicate files on drive and 409 errors on
amazonclouddrive however it will slow down the upload slightly as
another roundtrip will be needed.
None of the other Fses needed adjusting.
Fixes#483
