int create_user(const char *name, User **user_ptr_add) {

    User *tail, *new_user;
    
    // if entered user already exists in this list
    for (tail = *user_ptr_add; tail != NULL; tail = tail->next) {
        if (strcmp(tail->name, name) == 0) {
            return 1;
        }
    }

    // if given name cannot fit in the 'name' array
    if(strlen(name) >= MAX_NAME) {
        return 2;
    }

    // allocating space for new user 
    new_user = malloc(sizeof(User));
    if (new_user == NULL) {
        perror("Space for user not allocated properly");
    }

    // init a new user
    strncpy(new_user->name, name, MAX_NAME - 1);
    new_user->name[MAX_NAME - 1] = '\0';
    new_user->first_post = NULL;
    for (int i = 0; i < MAX_FRIENDS; i++) {
        new_user->friends[i] = NULL;
    }
    new_user->next = NULL;

    // inserting new user at tail of linked list
    if (*user_ptr_add == NULL) {
        *user_ptr_add = new_user;
    }
    else {
        tail->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 (don't forget about the null terminator)