Add \keys_set_known:nn

It seems sensible not to require a tl here in cases where people do not want
to save the information on unused keys. At the same time, a minor 'style' 
improvement to the return code, such that the clist is properly converted into
a tl.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4545 de43f980-851b-0410-b2f7-c40aca1f87e0