used wrong var name in docs

closees PR 13985
normalize-path can raise non-exn:fail:filesystem? exceptions
when this function should still be returning #f (and not
raising an exn)
move the with-stack-checkpoint wrapper code into the eval handler
so tools that override the handler can get direct access to the
unexpanded syntax objects if they want them
fix another bug introduced in efd2683301
move the teachpack/deinprogramm collection into
the deinprogramm pkg

This required making the deinprogramm pkg into a 'multi
pkg, which is why all of those other files moved
around too