(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.

src/lib/global/symbol.c

@@ -94,6 +94,7 @@ struct symbol meta_type[]  = {
     {NSC_INT, "d"},
     {NSC_XCOORD, "d"},
     {NSC_YCOORD, "d"},
+    {NSC_HIDDEN, "d"},
     {NSC_TIME, "d"},
     {NSC_FLOAT, "g"},
     {NSC_STRINGY,"c"},
@@ -124,6 +125,7 @@ struct symbol nation_flags[] = {
 };
 
 struct symbol nation_relations[] = {
+    {-1, "unknown"},
     {AT_WAR, "at-war"},
     {SITZKRIEG, "sitzkrieg"},
     {MOBILIZATION, "mobilization"},