Exit successfully with help-related options.

This PR makes `dfu-programmer` exit with success when using options like `--version` and `--targets`.  It also cleans up some remnants of the version command.  An alternative strategy might be to simply use `exit(0)` rather than communicating to the top-level the way this PR does.

If you need me to tweak anything, let me know.  I don't mind whipping it into a form that you guys are happy with.

Thanks for the excellent work!