Make comment more meaningful for sum/hashs between mtree_writer
and reg_info (former is used as transient storage for computation,
the latter to print out the final result for the file entry).

Rename "seed" to "crc_total", as seed overloads crc_total in
original mtree.

Implement the crc_total computation.
Add a dummy fprintf() function to print out the CRC total to see
whether the computation is correct.

XXX this ought to move to mtree binary instead of libarchive's format.