Efficiency improvements in l3file functions

These weere suggested by Qing Lee: more or less the same idea as in LaTeX2e
(find the file and reuse the name for loading). Documentation currently
only allows internal reuse of the name: this seems reasonable as there will
soon be an interface for 'input if exists'.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5168 de43f980-851b-0410-b2f7-c40aca1f87e0
Rearrange some docs

Presumably these will go into l3tl!


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5169 de43f980-851b-0410-b2f7-c40aca1f87e0