Reorder &rtp to have default values in sensible places

Use the default value of &rtp to be operating system independant.  Move
the /after directories at the end and the other default directories at
the beginning of &rtp.

Known issues
* If a user removes one of the default directories from &rtp it will be
  added again.
* If a user added some other directory manually it might end
  up in an unintended place.