Rich_text: do not silently truncate buffers, but export function [truncate].
Log: uses [Rich_text.truncate] on messages printed on terminal, and prefix
     truncated messages to indicate the truncation.

(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.