(nsc_type): New member NSC_HIDDEN.

(nsc_type): New member NSC_HIDDEN.
(meta_type): Add its entry.
(setnum, nstr_promote, nstr_exec_val): Deal with it.  nstr_exec_val()
implements opt_HIDDEN by mapping unknown values to -1.
(natstr, nat_ca): Use it for member nat_relate.  This also halves its
size.  Fixes very minor leak: before, player could see relations to
all countries, regardless of contact.
(nation_relations): Add entry for -1.