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.