fs: Implement PartialUploads feature flag

Implement a Partialuploads feature flag to mark backends for which
uploads are not atomic.

This is set for the following backends

- local
- ftp
- sftp

See #3770

backend/sftp/sftp.go

@@ -994,6 +994,7 @@ func NewFsWithConnection(ctx context.Context, f *Fs, name string, root string, m
 	f.features = (&fs.Features{
 		CanHaveEmptyDirectories: true,
 		SlowHash:                true,
+		PartialUploads:          true,
 	}).Fill(ctx, f)
 	// Make a connection and pool it to return errors early
 	c, err := f.getSftpConnection(ctx)