dot: add option 'A' to disable 'a'

This way in 2.5 we can make 'a' the default, and tell people to use
SPOT_DOTDEFAULT=A if they want the old behavior in both 2.4 and 2.5.

* spot/twaalgos/dot.cc: Implement the option.
* NEWS, bin/common_aoutput.cc: Mention it.
* tests/core/readsave.test: Test it.

spot/twaalgos/dot.cc

@@ -187,6 +187,9 @@ namespace spot
             case 'a':
               opt_show_acc_ = true;
               break;
+            case 'A':
+              opt_show_acc_ = false;
+              break;
             case 'b':
               opt_bullet = true;
               opt_bullet_but_buchi = false;