bsd-family-tree: add DragonFly 6.0.0

Tagged on May 8, 2021.

Sponsored by: Netflix
Approved by: re@ (delphij@)

(cherry picked from commit 779b70a2264fa187beb232dad2f018d41c31e34a)
(cherry picked from commit 79afe5598283eaf34cd3dcac5e78e64738d5f592)