rogue4: add -n option and system savedir

rogue4/save.c

@@ -197,6 +197,12 @@ char **envp;
 
     if ((inf = open(file, 0)) < 0)
     {
+        if (use_savedir && errno == ENOENT)
+        {
+            /* We're using a system savefile which doesn't exist.
+               This isn't a fatal error, it means start a new game. */
+            return TRUE;
+        }
 	perror(file);
 	return FALSE;
     }