Add 'contrib/spleen/' from commit '5eab6333fa27e2b6954c6927859d462a004e57bb'

git-subtree-dir: contrib/spleen
git-subtree-mainline: 3582acbad303bb69895457a8a7dae0f485efe8ea
git-subtree-split: 5eab6333fa27e2b6954c6927859d462a004e57bb

Reviewed by: imp [earlier]
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20653