simplify math for arc4random_uniform() suggested by Jinmei_Tatuya AT isc.org via jakob@
simplify math for arc4random_uniform() suggested by
Jinmei_Tatuya AT isc.org via jakob@

empirically verified for entire domain of upper_bound

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars