Actually make I18N::Langinfo avail on all platforms

Actually make I18N::Langinfo avail on all platforms

I thought I had done this earlier, but testing on Windows demonstrated
that I hadn't.

While at it, move the details in the docs for Perl_langinfo to the
module's pod.

This doesn't follow the paradigm for putting the Configure stuff in all
the related configure files, but I saw no point to doing so.  If you are
reading this because I was wrong, feel free to ticket it or fix it.