kqueue: Add NOTE_REAP

Add a NOTE_REAP event for EVFILTER_PROC which provides a notification
when the process is reaped.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D58313

(cherry picked from commit 2bacbbecb165dd761ea7ec2fc35630db61508cdf)