This commit allows the working directory for the (old) Server
implementation to be changed without doing a `os.Chdir` first.

The feature can be enabled with `sftp.WithServerWorkingDirectory(dir)`
passed as an option to `sftp.NewServer`.

It is useful when the `sftp` is used as part of a larger service that
does more than just serve `sftp` and using `os.Chdir` is not an option.

The fallback behavior (when the option is not specified) is that the
path remains unmodified (as before).