admin/store/settings/cart displays wrong units for cart duration

cf. http://www.ubercart.org/issue/10840/wrong_duration_cart_settings_overview

Authenticated cart duration units were being taken from the variable 'uc_cart_anon_unit' instead of 'uc_cart_auth_unit'. Bug affects only the display of the settings on the cart settings overview page - settings are stored properly in the DB and are properly applied in the code (hook_cron()), so no hook_update() is needed. Patch attached.