rename main() to something like driver()

[mcarifio]

I'd like to repurpose memtester to use it as a python module via cffi. It would be useful to refactor main() into a driver() similar to the existing main() and driver() that drives the iteration, backoff algorithm and output. Would also like a variant that generates json output to be consumed by some caller and additionally processed. The Makefile might need modification as well. I'll try my hand at a pull request, but thought I'd just mention these ideas.