Improve dvips driver set up/explanation

Quite a lot of this is opaque, so sorting out why the various bits of code
are required is important. See http://tex.stackexchange.com/q/152270 for more
on the fact that dvips does some 'interesting' things with the units used.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4637 de43f980-851b-0410-b2f7-c40aca1f87e0