Re-applying Sanitize path names

[fclairamb]

Re-applying #435:

In particular, guards against the use of backslash delimited filepaths as used on Windows.

This situation can occur when nesting s3.Fs inside an afero.BasePathFs which will process paths using filepath package.

On Windows calls to MkdirAll will use Windows paths and will create backslash paths on S3 which is undesirable.

Fixes https://github.com/fclairamb/afero-s3/issues/434

Signed-off-by: Jack Mordaunt [email protected]