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

examples/d/calc/calc.y

@@ -53,7 +53,7 @@ input:
 line:
   EOL
 | exp EOL           { writeln ($exp); }
-| error EOL
+| error EOL         { yyerrok(); }
 ;
 
 exp:

examples/d/simple/calc.y

@@ -51,7 +51,7 @@ input:
 line:
   EOL
 | exp EOL           { writeln ($exp); }
-| error EOL
+| error EOL         { yyerrok(); }
 ;
 
 exp:

tests/calc.at

@@ -720,7 +720,7 @@ exp:
 | "-" exp  %prec NEG { $$ = -$2; }
 | exp "^" exp        { $$ = power ($1, $3); }
 | "(" exp ")"        { $$ = $2; }
-| "(" error ")"      { $$ = 1111; }
+| "(" error ")"      { $$ = 1111; yyerrok(); }
 | "!"                { $$ = 0; return YYERROR; }
 | "-" error          { $$ = 0; return YYERROR; }
 ;