Save/restore the PRGS at checkpoints (necessary for internal demons).

This is necessary because I cannot Dynlink .cmxs files from a ocaml
toplevel. Hence, one way to load the algos from rdbg is to
#load .cma files. This option generates an ocaml file that
can be #used by automatically generated rdbg-session.ml files.

Rationale: it was not really necessary (Algo.neighbor contains almost
the same information), and the Topology.neighbor type was alone in its
module, which looked weird.

Depending on the kind of network, we should raise an exception. Indeed
such information should not be available in anonymous networks.

We will add such checks later.

instead of duplicating its content

Fix: in the generated lutin demon, the constraint to prevent several actions to be activated in the same process was wrong

Actually, I've just realized that the I/O mode of rdbg already makes
it possible to use sasa from rdbg (exit event only). This plugin would
only be necessary when we would need a finer grained intrumentation, e.g.,
to stop at enable step, or to at each process call/exit.

with algo several actions, and allow lurette to chose them.

and reuse it for Neighbor, which make the naming more consistent.

Also add the list of neighbors to the init function. The rationale is that
to give a sensible initial value to local vars of type 'Neighbor', we need
the neighbors number.

I could have chosen to pass an integer only. Well, who can the plus
can the minus.