model-checker: Add more progress functions.

model-checker: Add more progress functions.

The model checker supports "progress functions" that report the current
status of the model checking run.  Until now the implementation only
exported a single progress function that printed a line of dots across
stderr.  This commit moves the "fancy" progress function that was
previously part of the PSPP language code into the model checker itself
and adds an even more verbose progress function as well.