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. The users are stored in a linked list
int create_user(const char *name, User **user_ptr_add) { 
    if (strlen(name) >= MAX_NAME){                       
        return 2;           // given name can't fit in the 'name' array
    }
    User *curr = *user_ptr_add;
    while (curr != NULL){
        if (strcmp(name, curr->name) == 0){
            return 1;       // user with the name already exists
        }
        curr = curr -> next;
    }
    struct user new_user;
    strncpy(new_user.name, name, MAX_NAME - 1);
    curr = &new_user;
    return 0;
}
