(parents of handicapped minor child sold home to child through child's guardian who used child's funds to satisfy mortgage on home; guardian transferred title of home to himself as child's guardian and claimed first-time homebuyer tax credit; credit disallowed because child deemed, under step-transaction doctrine, to have purchased home from related persons under Sec. 36(c)(5); no analysis of Sec. 36(c)(3) which does not bar purchases for fair market value from related persons).