Add string_in_list() and add_string_to_list() with tests
    
    string_in_list() and add_string_to_list() are two utilities for NULL
    terminated strings arrays. add_string_to_list() adds a new string to an
    existing list or creates a new one with the strings as only item if
    there is not list. string_in_list() checks if a given string is in the
    list. It can be used case sensitive or in-sensitive.