Merge reaper facility.

MFC r270443 (by mjg):
Properly reparent traced processes when the tracer dies.

MFC r273452 (by mjg):
Plug unnecessary PRS_NEW check in kern_procctl.

MFC 275800:
Add a facility for non-init process to declare itself the reaper of
the orphaned descendants.

MFC r275821:
Add missed break.

MFC r275846 (by mckusick):
Add some additional clarification and fix a few gammer nits.

MFC r275847 (by bdrewery):
Bump Dd for r275846.