o Use ng_timeout() instead of timeout(9).
o Use ng_timeout() instead of timeout(9).

o Implement some netgraph flow control:
- Whenever status of HDLC heartbeat from pear is timed out,
  send NGM_LINK_IS_DOWN message.
- If HDLC link changes status from down to up, send
  NGM_LINK_IS_UP message.

Approved by:	julian (mentor), implicitly
MFC after:	1 week

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