Create DVDs with xorriso
    
    Use a different approach for building DVDs when xorriso is enabled.
    
    The default of using genisoimage is not changed at all. When the config
    option is set to use xorriso, the actual execution is different between
    bootable and non-bootable images.
    
    The non-bootable images are still created by running xorrisofs (which is
    a compatibility tool with same UI as genisoimage). Since the image is
    not bootable, there should be no problems with boot options.
    
    For bootable images, Pungi will instead take the boot.iso generated by
    Lorax, and use xorriso to inject all the extra files into the image.
    
    The shell script that used to invoke all the commands to build the ISO
    now runs the `xorriso` command in interactive mode and feeds another
    file into it. The new file contains the xorriso commands to add the
    required files to the image.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>