- [cacheable] is given at the registration of a builtin, and not separately
  for each result of the builtin.
- Removes the offsetmap for the arguments: no builtin were using it anyway.
- Returns a Cvalue.V.t for the result instead of an offsetmap.
- Builtins can also return:
  + only the value of the result, in which case the post-state from the
    interpretation of the specification is used with the resulting value
    computed by the builtin.
  + only the post-states for functions with no froms dependency (other than the
    result and arguments) and no write of local variable addresses.

Simplifies some builtins accordingly.

Type [cacheable] is now defined in Eval.
Type [call_result] is defined in Builtins.
Changes the type of callbacks Db.Call_Type_Value_Callbacks.

This function links the kernel functions to their builtins for a given analysis.
Checks that the builtin replacing a function has a compatible type and a
specification. These requirements are not checked each time a builtin is used.

The type of a builtin is now a function "unit -> typ" applied when the analysis
starts, allowing builtins to use types defined from the machdep, such as size_t.

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.

This module type simplifies the signature of functors using evaluation
functions.

(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.