MFC 253887,253977:

- Use pget(PGET_CANDEBUG | PGET_NOTWEXIT) to determine if the specified
  PID is valid for monitoring in FILEMON_SET_PID ioctl.

- Set the monitored PID to -1 when the process exits.

Suggested by:	jilles
Tested by:	sjg
Clearing the rxbuf flags got lost along the way... adding it back.