(disconnect component detection)

The previous  scheme was to  define everything  that is common  to all
user nodes in  a file named state.ml. This file was supposed to define
the algo  state type,  as well  as optional  functions such  as fault,
legitimate, or potential.

But in  order to be  able to define,  say, a legitimate  function that
uses user  nodes (which is the  case for dijkstra token  ring) without
introducing any dependency loop, it  is necessary to split state.ml in
two modules.

The new module is called Config (config.ml).

nb: I had to modify the GenSessions module in rdbg to reflect this change !