Fall back to old "stock_person" icon if avatar-default is unavailable
    
    Older versions of gnome-icon-theme don't have avatar-default.  This
    commit makes gdm fallback to the icon it previously used for faces,
    if avatar-default isn't around.
    (cherry picked from commit 0dd6e3f8dfc0419146d3ca1fc045e84a73ecf6ec)