In domains, changes [Store.register_global_state] that now takes a boolean
argument indicating whether the domain states must be saved during the
analysis.

In the function [initialize_variable_using_type].
Adds a constructor [Return] to [variable_kind] for the special variable that
stores the return value of a function call.

These module definitions were previously in abstract_value, abstract_location
and abstract_domain. They are now grouped in this new file for the three
abstractions.
These module types should only be useful for the engine, and need not to be
visible in the implementation of the various abstractions.

New module type Leaf in abstract_value, abstract_location and abstract_domain
with the key identifying the module. The internal structure is no longer needed
in these abstractions, and is instead built by the engine from this key.

Keys are no longer exported outside the modules.
For abstract domains, the key is automatically created by the functor
Domain_builder.Complete from the domain name.

Many changes throughout the analyzer:
- in the engine, Abstract_value.{Internal|External} becomes
  Abstract.Value.{Internal|External}.
- in the abstractions, Abstract_value.Internal becomes Abstract_value.Leaf.

(blind make headers from specifications)

- Fix Value/Value#5
- Fix Value/Value#14

Some case studies (e.g. dyad) use some ugly casts from fd_set_t which lead
to the analysis stopping too early.
Changing the representation of fd_set_t should also help it better conform to
the standard (since a fd_set_t should be able to hold FD_SETSIZE elements).

[Ptests] preserve LOG after STDOPT directive

See merge request frama-c/frama-c!2073

This is absolutely not a sneaky attempt to relaunch a build (now that OCI seems
in better shape) pushing a nearly empty commit.