Fix race between a process registering a NOTE_EXIT EVFILT_PROC event and

Fix race between a process registering a NOTE_EXIT EVFILT_PROC event and
the target process exiting which causes attempts to register the kevent
to randomly fail depending on whether the target runs to completion before
the parent can call kevent(2).  The bug actually effects EVFILT_PROC
events on any zombie process, but the most common manifestation is with
parents trying to monitor child processes.

MFC after:	2 weeks
Sponsored by:	NTT Multimedia Communications Labs