Honor *nix umask when downloading files to local machine

[ruixingw]

Under macOS or Linux, files downloaded to local machine by coscli cp or coscli sync always have 640 permission.

I understand that the Cloud Object Storage doesn't store *nix file permission, so coscli has to assign one when downloading files to *nix systems. However, I think it is better to honor umask instead of hardcoding a 640 permission.

By the default umask 0022, files downloaded by coscli would have 755 permission.