The provisions of this part restricting the possession and control of controlled substances shall not apply to common carriers or to warehousemen, while engaged in lawfully transporting or storing such substances, or to any employee of the same acting within the scope of his employment; or to public officers or employees in the performance of their official duties requiring possession or control of controlled substances; or to temporary incidental possession by employees or agents of persons lawfully entitled to possession or by persons whose possession is for the purpose of aiding public officers in performing their official duties.