Fix origin and zdone for invalid country argument

Invalid country argument was interpreted as (natid)-1, which is
normally 255.  Harmless as long as this is less than MAXNOC (99).
Wrong anyway.

src/lib/commands/zdon.c

@@ -59,7 +59,7 @@
 int
 zdon(void)
 {
-    natid whichcnum;
+    int whichcnum;
     struct natstr *natp;
     char *p;