liveimage-mount: Add an unmount mode.
    
    Add an unmount mode for undoing a previous invocation that
       used the --persist option.
    Add utility functions to find intermediate mount points, loop
       devices and their backing files, and device-mapper targets.
    Add cleanup and exit functions.
    Update the usage statement (now modeled after argparse).
    
    Also, add support for symlinks and more error checking.
    Support rootfs.img & ext3fs.img as names for the LiveOS root
       filesystem image file.
    Create a mount point directory if nonexistant.
    Cleanup some long lines and awkward code.