Don't upload `partials` to permanent

[slifty]

Rclone's sftp will, by default, upload a partial file and then rename. Our sftp implementation doesn't support renames, which means that this approach will result in the creation of a bunch of .partial files.

This isn't a good user experience.

We should either:

1. Error when we are asked to create a .partial, with a message instructing the user to not use partials (--inplace).
2. Build Permanent API support for rename and therefore add support for SFTP rename. (Support SSH_FXP_RENAME #164)