Retroactively add a version number check before returning BOOTPART=
From the comment: * "BOOTPART=" confuses the old rc.frisbee argument parsing * which looks for "PART=" with the RE ".*PART=" which will * match BOOTPART= instead. Thus an old script loading a * whole disk image (PART=0) winds up trying to load it in * partition 2 (BOOTPART=2). So we can pick one of two * versions, the one in effect when rc.frisbee changed its * argument parsing (v30, circa 6/28/2010) or the version * in effect when BOOTPART was added (v36, circa 6/13/2013). * We choose the latter.