Fixes a performance issue.

- shares and moves the functions [reduce_offset_by_validity] of Locations and
  Precise_locs into base.ml.
- replaces the boolean argument ~for_writing into the new type access, that
  represents Read, Write or No_access. Without any access, offsets must point
  into or just beyond the base validity.
- fixes the support of accesses of size 0: they are now invalid:
  + in bases with Invalid validity;
  + one past a base validity unless the base ends with an empty struct.

(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.