Some simple changes to get Drush 6 to recognize Backdrop as Drupal 7.

Messing around to see what would be involved with getting Backdrop to install in Aegir, I came up with this simple solution: https://drupal.org/node/2094221#comment-7953811. Basically, just symlink some directories, so Drush 6 recognizes Backdrop as Drupal 7, and change the version strings.