Use a flag-based approach for the 'skip unknown keys' concept

Introducing groups as well means that there will be several potential
paths though the code. Using a flag makes it easier to keep the number of
internal routes managable.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4548 de43f980-851b-0410-b2f7-c40aca1f87e0