extract ISO feature: make sure *.squashfs files exist + support older ISOs too

ISOs before release 2011.12 used /live/ for storing squashfs files,
releases starting with 2011.12 use /live/*/ (where "*" corresponds
to the release name). Test whether such files exist before accessing
them and while adding the check also add support for releases older
than 2011.12.

Thanks: Charles A. Hewson <cahewson@eskimo.com> for the bugreport