int create_user(const char *name, User **user_ptr_add) {
    // ptr is pointer to a pointer to a struct
    // we are basically creating a new node in a linked list
    // to create a new node, we need the node that follows it
    // structs are not passed to functions in the ame way as arrays; we don't pass a pointer by default
    if(strlen(name) > (MAX_NAME + 1)) {return 2;} //stlen does not consider null terminator
    //Now, let's check if the name already exists: let's travense through the
    // linked list and check for the name of every other user:
    User* curr = *user_ptr_add;
    while (curr->next!= NULL)
    {
        if(strcmp(curr->name, name)){return 1;}
        curr = curr->next;
    }
    
    User* new_user = malloc(sizeof(User)); //allocate space for new user
    strcpy(new_user->name, name);
    new_user->next = NULL;
    *curr->next = *new_user;
    return 0;
}
Create a new user with the given name.  Insert it at the tail of the list
 * of users whose head is pointed to by *user_ptr_add.
 
 * Return:
 *   - 0 if successful
 *   - 1 if a user by this name already exists in this list
 *   - 2 if the given name cannot fit in the 'name' array
 * 