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)