Backport Drupal 6 $body_classes

Drupal 6 will have new classes in its $body_classes variable: http://drupal.org/node/171906

Zen should make its $body_classes include all of the same classes.

Specifically we will have to change:

• "ntype-" with "node-type-"
• "both-sidebars / sidebar-left / sidebar-right" with "two-sidebars / one-sidebar sidebar-left / one-sidebar sidebar-right / no-sidebars"