do not expect/set a trailing “/” in CFG_DIR (cosmetic)

* Usually it’s dangerous to expect users to end directories at configuration options with “/”.
  Therfore added the “/”-separator there where it’s actually needed (when the directory is concatenated with a file).
* Removed the auto-appended traling “/” at the initialisation of CFG_DIR with the configure macros.
  At later places, the separating “/” was already used cleanly, and there this would have lead to double “//”, which is kind of ugly in log output and that like.