fssum: seek for data before adding to the sum

This change is incompatible with previous versions of fssum and changes all
sums computed with the default option to include file data (-d/-D switch).

To compute sums for large sparse blocks, we use SEEK_DATA before reading. To
detect holes of different size, each data block is preceeded by its offset
in the file. Holes are not included in the sum explicitly.
1 file changed