(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/nat.h

@@ -114,7 +114,7 @@ struct natstr {
     time_t nat_newstim;		/* date news last read */
     time_t nat_annotim;		/* date annos last read */
     float nat_level[4];		/* technology, etc */
-    short nat_relate[MAXNOC];
+    unsigned char nat_relate[MAXNOC];
     unsigned char nat_contact[MAXNOC];
     unsigned char nat_rejects[MAXNOC];
     signed char nat_priorities[PRI_MAX+1]; /* budget priority */