Don't emit a spurious EVFILT_PROC event with no fflags set on process exit
if NOTE_EXIT is not being monitored. The rationale is that a listener
should only get an event for exit() if they registered interest via
NOTE_EXIT. This matches the behavior on OS X.

• Don't save the exit status on process exit unless NOTE_EXIT is being monitored.
• Add an internal EV_DROP flag that requests kqueue_scan() to free the knote without signalling it to userland and use this when a process exits but the fflags in the knote is zero.

Reviewed by: jmg
MFC after: 1 month