Allow estimate-difficulty to be exported (in Linux/macOS/Windows) as
independent executable, so that it can be used without having to install
Frama-C.

Remove obsolete 'minimal Python version' guards, which are likely insufficient,
due to the difficulty in testing them automatically. A better solution would
be useful.

(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.