File permissions on Windows

When packing a squashfs filesystem on Windows using `gensquashfs.exe`, all the files are created with 0644 permissions, even when `-d mode=0755` is set. This is problematic if the file system is designed to be mounted on Linux with FUSE and run. (in that instance, it will be read-only, and the permissions can not be corrected later) I don't think this is what the `--defaults` arg was intended for, but the help text seems to imply the `-d` default arg should be used for anything implicit. On windows, there is no file permissions per say, so I think it makes sense. Alternatively, we could have a separate argument allowing us to override the permission of all files? Even better would be an glob, because technically only the executable files need to be set to 0755.