TODO to support full MP handling with multiple connections.
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some of the following values must be changed to fields: 
(put them into the link structures)
  xmit_accm
  (kill_link, is global)
  lcp_echos_pending
  lcp_echo_fails
  lcp_echo_number
  lcp_echo_interval
  idle_time_limit
  lcp_echo_timer_running
  hungup
  (baud_rate isn't necessary)

MPmrru and mru handling