Java: add support for parse.error custom|detailed

[akimd]

This is my proposal to add support for parse.error custom|detailed to Java. I'm no Java programmer, and I have absolutely no idea if this is the proper way to do it (and push parsers must be checked too). But at least it's a starting point from which to discuss.

To see it work, have a look at examples/java/calc:

%define parse.error custom
...
%code {
  static String _ (String s)
  {
    return s;
  }
}
%%
class CalcLexer implements Calc.Lexer {
...
  public void yyreportSyntaxError (Calc.Context ctx)
  {
    final int ARGMAX = 10;
    int[] arg = new int[ARGMAX];
    int n = ctx.yysyntaxErrorArguments (arg, ARGMAX);
    System.err.print (ctx.yylocation + ": syntax error");
    for (int i = 1; i < n; ++i)
      System.err.print ((i == 1 ? ": expected " : " or ")
                        + ctx.yysymbolName (arg[i]));
    if (n != 0)
      System.err.print (" before " + ctx.yysymbolName (arg[0]));
    System.err.println ("");
  }
...