skeletons: add support for %code epilogue

When building the test cases, emitting code in the epilogue is very
constraining.  Let's make it simpler thanks to %code epilogue.

However, I don't want to document this: it is bad style to use it (we
should avoid having too many ways to write the same thing,
TI!MTOWTDI), just put your code in the true epilogue section.

* data/skeletons/glr.c, data/skeletons/lalr1.d, data/skeletons/lalr1.java,
* data/skeletons/yacc.c: Implement support for %code epilogue.
Remove useless comments.
* tests/calc.at, tests/java.at: Simplify.

data/skeletons/glr.c

@@ -2720,5 +2720,6 @@ m4_if(b4_prefix, [yy], [],
 #define yynerrs ]b4_prefix[nerrs]b4_locations_if([[
 #define yylloc  ]b4_prefix[lloc]])])[
 
-]b4_epilogue[]dnl
+]b4_percent_code_get([[epilogue]])[]dnl
+b4_epilogue[]dnl
 b4_output_end

data/skeletons/lalr1.d

@@ -902,9 +902,8 @@ m4_popdef([b4_at_dollar])])dnl
       stream.writeln ();
     }]])[
   }
-
-  /* User implementation code.  */
 ]b4_percent_code_get[
 }
-]b4_epilogue[]dnl
+]b4_percent_code_get([[epilogue]])[]dnl
+b4_epilogue[]dnl
 b4_output_end

data/skeletons/lalr1.java

@@ -1036,10 +1036,8 @@ b4_dollar_popdef[]dnl
   private static final int yyfinal_ = ]b4_final_state_number[;
   private static final int yyntokens_ = ]b4_tokens_number[;
 
-/* User implementation code.  */
-]b4_percent_code_get[]dnl
-
+]b4_percent_code_get[
 }
-
+]b4_percent_code_get([[epilogue]])[]dnl
 b4_epilogue[]dnl
 b4_output_end

data/skeletons/yacc.c

@@ -2065,5 +2065,6 @@ yypushreturn:]])[
     YYSTACK_FREE (yymsg);]])[
   return yyresult;
 }
-]b4_epilogue[]dnl
+]b4_percent_code_get([[epilogue]])[]dnl
+b4_epilogue[]dnl
 b4_output_end