Permit no-op country name change again

Commit aa5861d1 (v4.3.20) made add, edit and change reject a country
name that is already in use.  Even if it's used by the same country.
Relax that.

src/lib/commands/add.c

@@ -28,6 +28,7 @@
  *
  *  Known contributors to this file:
  *     Steve McClure, 2000
+ *     Markus Armbruster, 2004-2011
  */
 
 #include <config.h>
@@ -84,7 +85,7 @@ add(void)
     p = getstarg(player->argp[2], "Country name? ", buf);
     if (!p)
 	return RET_SYN;
-    if (!check_nat_name(p))
+    if (!check_nat_name(p, coun))
 	return RET_FAIL;
     strcpy(cntryname, p);
     p = getstarg(player->argp[3], "Representative? ", buf);