Set the p_oppid field of orphans when exiting.

Set the p_oppid field of orphans when exiting.

Such processes will be reparented to the reaper when the current
parent is done with them (i.e., ptrace detached), so p_oppid must be
updated accordingly.

Add a regression test to exercise this code path.  Previously it
would not be possible to reap an orphan with a stale oppid.

Reviewed by:	kib, mjg
Tested by:	pho
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D19825