Add pdfTeX support to l3unicode-data

As pdfTeX can be covered by a simple 'up front' test which bails-out of
loading the file, it seems easiest to handle all of this in l3unicode-data.def.
At present there is only the code to handle case-folding, but adding support
for other mappings should be easy enough.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5154 de43f980-851b-0410-b2f7-c40aca1f87e0