int make_post(const User *author, User *target, char *contents)
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 */