Don't show shutdown options when not available
    
    The logic for showing the shut down menu was a little
    iffy before.  This is apparently causing issues on solaris,
    and probably elsewhere.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=624298
    (cherry picked from commit d8a75a806215cf95832de70da7fdb9c7592d697f)