java: add support for lookahead correction

Shamelessly stolen from Adrian Vogelsgesang's implementation in
lalr1.cc.

* data/skeletons/lalr1.java (yycdebugNnl, yyLacCheck, yyLacEstablish)
(yyLacDiscard, yylacStack, yylacEstablished): New.
(Context): Use it.
* examples/java/calc/Calc.y: Enable LAC.

examples/java/calc/Calc.y

@@ -23,11 +23,17 @@
 %define api.parser.public
 %define api.push-pull push
 
+// Customized syntax error messages (see reportSyntaxError)...
 %define parse.error custom
-%define parse.trace
 
+// ... with locations...
 %locations
 
+// ... and accurate list of expected tokens.
+%define parse.lac full
+
+%define parse.trace
+
 %code imports {
   import java.io.BufferedReader;
   import java.io.IOException;