Reparent a child of pdfork(2) to its reaper when the procdesc is closed.

Unconditionally reparenting to PID 1 breaks the procctl(2) reaper
functionality.

Add a regression test for this case.

Reviewed by: kib
Approved by: re (gjb)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17589