- Implement prop_{array,dictionary}_copyout(), based on
  prop_{array,dictionary}_copyout_ioctl().
- Implement prop_{array,dictionary}_recv_syscall(), which
  takes as parameter the pref we got from kernel and internalize it,
  and unmaps the buffer prop_{array,dictionary}_copyout() mapped for us.
- add a prop_{array,dictionary}_send_syscall() for symetry, which is
  an alias to prop_{array,dictionary}_externalize_to_pref()

Discussed on tech-kern@ and tech-userlevel@

(bouyer)