/* 
 * 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 
 */  