NOTE_TRACK, EVFILT_PROC, kqueue, and subreapers

[Jonathan de Boyne Pollard]

Jilles Tjoelker:
> This should probably be fixed. 

There's another more insidious bug hiding inside kevent() somewhere that 
causes a kernel abend complaining about a sleeping thread holding a 
non-sleepable lock.  One needs to make fairly heavy use of kevent() in 
order to trigger it, I believe, as I have only seen it so far on systems 
that do.