Simplify the C++ parser constructor.

* data/lalr1.cc (debug_): Rename as...
(yydebug_): so that the parser's internals are always in the yy*
pseudo namespace.
Adjust uses.
(b4_parse_param_decl): Remove the leading comma as it is now only
called as unique argument list.
(Parser::Parser): Remove the constructor accepting a location and
an initial debugging level.
Remove from the other ctor the argument for the debugging level.
(debug_level_type, debug_level, set_debug_level): New.
* tests/actions.at, tests/calc.at, tests/regression.at: Adjust
constructor calls.

tests/regression.at

@@ -735,7 +735,8 @@ yy::Parser::error_ ()
 int
 yyparse (void)
 {
-  yy::Parser parser (!!YYDEBUG);
+  yy::Parser parser;
+  parser.set_debug_level (!!YYDEBUG);
   return parser.parse ();
 }
 ],