Move L3-specific build settings to a separate config Lua file

Hopefully this means most (all?) of l3build.lua is now not tied to our
own work. Still a few things to generalise in terms of settings.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4989 de43f980-851b-0410-b2f7-c40aca1f87e0
Rename "pdfsettings" to "typesetcmds"

Hopefully this is more clearly linked to typesetting.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4990 de43f980-851b-0410-b2f7-c40aca1f87e0