l3docstrip: add module replacement for single-line guards

It didn't seem necessary to pass an argument into \replaceModuleInLine
to use a different scratch variable than \inLine in this case

closes #164

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4785 de43f980-851b-0410-b2f7-c40aca1f87e0