[tlbuild] update to xetex - please rebuild if possible

[Karl Berry]

    is functionally identical except for the potential non-portability of  
    formats, so it isn't a critical issue.

Actually, it is a critical issue, since otherwise sites with
multiplatform installs are dead in the water.

Coincidentally (?), we also just discovered that pdftex .fmt's were
likewise unportable (for completely different reasons).  A fix for that
has been committed.

But please hold off on rebuilds until we've done a bit more testing and
I issue another call.  Later today I hope.

Next year we will test this *first* :).

(BTW, LuaTeX .fmt's are also unportable; that won't be fixed for this
release, it's yet another separate issue, much deeper in that case --
using Lua bytecode, which is inherently system-dependent.)

Thanks,
k