int make_friends(const char *name1, const char *name2, User *head) {
    if (find_user(name1, head) == NULL || find_user(name2, head) == NULL) {
        return 4;
    } else if (strcmp(name1, name2) == 0) {
        return 3;
    } 

    User *user1 = find_user(name1, head);
    User *user2 = find_user(name2, head);
    for (int i = 0; i < MAX_FRIENDS; i++) {
        if (user1->friends[i] == user2) {
            return 1;
        } else if (user2->friends[i] == user1) {
            return 1;
        } 
    }

    if (user1->friends[MAX_FRIENDS - 1] != NULL || user2->friends[MAX_FRIENDS - 1] != NULL) {
        return 2;
    } else {
        int i = 0;
        while (user1->friends[i] != NULL) {
            i++;
        }
        user1->friends[i] = user2;

        int j = 0;
        while (user2->friends[j] != NULL) {
            j++;
        }
        user2->friends[j] = user1;
    }
    return 0;
}
 Make two users friends with each other.  This is symmetric - a pointer to
 * each user must be stored in the 'friends' array of the other.
 *
 * New friends must be added in the first empty spot in the 'friends' array.
 *
 * Return:
 *   - 0 on success.
 *   - 1 if the two users are already friends.
 *   - 2 if the users are not already friends, but at least one already has
 *     MAX_FRIENDS friends.
 *   - 3 if the same user is passed in twice.
 *   - 4 if at least one user does not exist.
 *
 * Do not modify either user if the result is a failure.
 * NOTE: If multiple errors apply, return the *largest* error code that applies.