Add a comment and FIXME that lualatex can make DVI

Add a comment and FIXME that lualatex can make DVI

The command 'lualatex' can produce a DVI with the option
--output-format=dvi
It is best to keep things as is because it is better to guess a PDF
than to guess a DVI (we do not use that feature of the 'lualatex'
command internally; we use 'dvilualatex' instead). However, we
should ideally get this information in a more robust way.

Thanks to Günter for pointing this out.