pwait: Optionally wait until process is reaped

If the new -r option is specified, wait until the target process not
only terminates but is reaped.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D58314

(cherry picked from commit c8f5e6819d4d81906c4a1641b5c9f02d8730481c)