define PR_SET_CHILD_SUBREAPER if not set

Do an explicit define of `PR_SET_CHILD_SUBREAPER` for system with older headers.

~~Currently, setting subreaper option will fail in rhel/centos 6.5/6.6, which causes `docker exec` to fail
https://botbot.me/freenode/docker-dev/2014-12-13/?msg=27459979&page=2 . This patch ignores errors casued by setting subreaper option.~~