Add a facility for non-init process to declare itself the reaper of
the orphaned descendants.  Base of the API is modelled after the same
feature from the DragonFlyBSD.

Requested by:	bapt
Reviewed by:	jilles (previous version)
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	3 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275800 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Fix typo.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275802 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f