Fix mount options for vfat filesystems on automounting (Unicode)

See the commit message for more info.

This change only affects automounting behaviour. The "manual" mounting through the Thunar file-manager continues to get the argument wrong. I'll study the [hal architecture](http://www.marcuscom.com/hal-spec/hal-spec.html) and see how I can tune the system even further. I'll send another pull request when I'm done.