Kill all descendants of the reaper, even if they are descendants of a
subordinate reaper.
Also, mark reapers when listing pids.
Reported by: Michael Zuo <muh.muhten@gmail.com>
PR: 223745
Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D13183