Dialup server system

listen on a given port to internal addresses.

when a connection is made:

• lock the refcountfile
• increase the refcount and check to see what the new value is
• if the new refcount was 1:
  • pppup
• ensure the connection is up
• unlock the refcountfile
• in any event, report the status of things over the connection

when a connection is dropped:

• lock the rcf
• decrease the refcount and check to see what the new value is
• if the new refcount was 0:
  • pppdown
• unlock the rcf