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

    struct user *curr_user = *user_ptr_add;
    while (curr_user->next != NULL) {
        if (strcmp(curr_user->name, name) == 0) {
            return 1;
        }
        curr_user = curr_user->next;
    }

    struct user *new_user = malloc(sizeof(struct user*));
    strcpy(new_user->name, name);
    curr_user->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.