* exit after providing an estimate if -S was given. the PR used -e,
but checking around indicated prior art in Solaris usin -S.
* remove superfluous 'DUMP:' prefix in two messages
* initialise blocksperfile explicitly (not necessary, but everything
else in that section gets initialised, so be consistant :)
* display the ``pretty'' name of the dumped directory, so the user
knows if it's a subset or not