Bugfix release and options -k added

Now large files (more than 2 GB) can be processed, one of my machines exposed that bug to me.

Checksumming a large file with options -l and -m I observed the problem, that long parts of NUL blocks always have an equal sum.  So the interleaving still does not protect against wrong ordering.

The complete sum will detect this, however this leaves a big puzzle on how to assemble all the pieces such that they are ok again.  This now is solved with the new option -k.

By mixing in the block number into the MD5 hash, the hashes become unique, such that - with interleaving - you can sort all block such that they fit where they belong.  (It's still a nontrivial task, but can be done.)

Option -k is followed by Option -p.