d: add yyerrok

In D's case, yyerrok() is a private method of the Parser class.
It can be called directly as `yyerrok()` from the grammar rules section.

* data/skeletons/lalr1.d: Add yyerrok().
* examples/d/calc/calc.y, examples/d/simple/calc.y: Demonstrate yyerrok().
* tests/calc.at: Update D tests to use yyerrok().

data/skeletons/lalr1.d

@@ -307,6 +307,11 @@ b4_user_union_members
   private static immutable YYSemanticType yy_semantic_null;])[
   private int yyerrstatus_ = 0;
 
+  private void yyerrok()
+  {
+    yyerrstatus_ = 0;
+  }
+
   /**
    * Whether error recovery is being done.  In this state, the parser
    * reads token until it reaches a known state, and then restarts normal