int create_user(const char *name, User **user_ptr_add) {
    if (strlen(name) > MAX_NAME - 1) {
        return 2;
    }

    if (*user_ptr_add == NULL) {
        User user;
        strcpy(user.name, name);
        *user_ptr_add = &user;
        return 0;
    }

    User *curr = *user_ptr_add;
    while (curr->next != NULL) {
        if(strcmp(curr->name, name) == 0) {
            return 1;
        }
        curr = curr->next;
    }
    User user;
    strcpy(user.name, name);
    curr->next = &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
 *       (don't forget about the null terminator)