Some checksum buffer fixes.

Some checksum buffer fixes.

- Put sum2_array into sum_struct to hold an array of sum2 checksums
  that are each xfer_sum_len bytes.
- Remove sum2 buf from sum_buf.
- Add macro sum2_at() to access each sum2 array element.
- Throw an error if a sums header has an s2length larger than
  xfer_sum_len.