Use libarchive helper functions in explodeRPM()
    
    Simplify the archive extraction loop in explodeRPM() by using the
    unpack.c helper functions.  Add a new parameter to explodeRPM, the
    destination to unpack the archive to.  This was previously handled
    by dlabelUnpackRPMDir() directly, but it made more sense to me to
    move it in to the libarchive unpack helper.