\keys_set_known:nnN should store in a tl

Bruno points out that while a clist is convenient for the implementation,
conceptually this is a tl.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4525 de43f980-851b-0410-b2f7-c40aca1f87e0
A label/ref pair missed in the 'add the module' sweep


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4526 de43f980-851b-0410-b2f7-c40aca1f87e0