fix multi FaceThread loss

Bug: starting ccnpingserver after ccnping may cause packet loss.
Observation:
  ccnpingserver's handle_prefix_reg_reply is called with INTEREST_TIMED_OUT
  FaceThread::Send and FaceThread::PullOutQueue see different payloads for same message
Root cause: both payloads are allocated from h->scratch_charbuf
Fix: make a copy of payload in stuff_and_send
  stuff_and_send is now in face/ccnd_interface.cc, and it won't stuff link message; link message is disabled