Added checkmark (CP437/Unicode).

This revealed a bug in the AlertBox code.
(Fixed with StringLen).

testdoor/testdoor.go

@@ -683,7 +683,7 @@ func main() {
 	if door.CP437 {
 		b = door.AlertBox("Warning: golang \xfb is in use!", 1)
 	} else {
-		b = door.AlertBox("Warning: golang \u2219 is in use!", 1)
+		b = door.AlertBox("Warning: golang \u221a is in use!", 1)
 	}
 
 	d.Write(door.ColorText("BRI WHI ON GREEN"))