Add \expanded to list of 'known' primitives

This could be added to either those from pdfTeX or LuaTeX: s pdfTeX 1.50 had
never been released, the more realistic position is that this is a LuaTeX
primitive.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4993 de43f980-851b-0410-b2f7-c40aca1f87e0