Tighten up the protected status of a few functions in l3keys

These seem to have been missed in earlier sweeps. Note in particular the
removal of a `\use:x` and switch there to a protected auxiliary. 


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4506 de43f980-851b-0410-b2f7-c40aca1f87e0