MFC 254287: Some small cleanups to the fixes in r180340:
- Set NOTE_TRACKERR before running filt_proc().  If the knote did not
  have NOTE_FORK set in fflags when registered, then the TRACKERR event
  could miss being posted.
- Don't pass the pid in to filt_proc() for NOTE_FORK events.  The special
  handling for pids is done knote_fork() directly and no longer in
  filt_proc().