[tlbuild] luatex once again

[Karl Berry]

Elie reported that io.popen in luatex was broken (everywhere but
Windows).  Since Akira came up with an instant and trivial fix (thanks
as always, Akira), and since io.popen is a pretty important routine in
LuaTeX, I think it's worth remaking.

I'm sure that not all platforms can be done in the next day or even two,
and that's ok, but whoever is around and can run it off, might as well ...

BTW, judging by the commit times, it seems that armel-linux and
i386-solaris/x86_64-solaris are lacking the latest mpost and dvisvgm
fixes, too.

Thanks,
Karl