int make_post(const User *author, User *target, char *contents) {
    if (author == NULL || target == NULL) {
        return 2;
    }
    for (int i = 0; i < MAX_FRIENDS; i++) {
        if (author->friends[i] == target) {
            Post *post = NULL;
            strcpy(post->author, author->name);
            strcpy(post->contents, contents);
            *post->date = time(NULL);
            if (target->first_post != NULL) {
                Post *temp = target->first_post;
                post->next = temp;
            }
            target->first_post = post;
        }
    }
    return 1;
}
Make a new post from 'author' to the 'target' user,
 * containing the given contents, IF the users are friends.
 *
 * Insert the new post at the *front* of the user's list of posts.
 *
 * 'contents' is a pointer to heap-allocated memory - you do not need
 * to allocate more memory to store the contents of the post.
 *
 * Return:
 *   - 0 on success
 *   - 1 if users exist but are not friends
 *   - 2 if either User pointer is NULL