s0artcl: make font selection compatible with XeTeX and LuaTeX

The basic example still complains about missing font shapes like:

    LaTeX Font Warning: Font shape `TU/DroidSans(0)/m/scsl' undefined
    (Font)              using `TU/DroidSans(0)/m/sc' instead on input line 45.

but in general the document looks similar enough as with pdflatex.