Sys.cmd is called _very_ often and thousands of times; whenever a Ctrl+C
is pressed while inside a subcommand, it returns a non-zero code. But
in many loops, a message is emitted and nothing else, so hdrck continues on
and on. Besides, the Ctrl+C leads to spurious "invalid header" messages at the
end, because of that. The fix simply makes the program terminate.

- 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.