(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.

include/nsc.h

@@ -57,6 +57,8 @@ typedef enum {
     NSC_INT,			/* int */
     NSC_XCOORD,			/* coord that needs x conversion */
     NSC_YCOORD,			/* coord that needs y conversion */
+    NSC_HIDDEN,			/* unsigned char in struct natstr that
+				   may need hiding */
     NSC_TIME,			/* time_t */
     NSC_FLOAT,			/* float */
     NSC_STRINGY,		/* char[], zero-terminated string */