Let the initial-action act on the look-ahead, and use it for the

"initial push" (corresponding to an hypothetical beginning-of-file).
And let lalr1.cc honor %initial-action.
* doc/bison.texinfo (Initial Action Decl): Clarify, and add an
example.
* data/lalr1.cc (Parser::initlocation_): Remove, bad experiment.
(Parser::Parser): Remove the ctor that used to initialize it.
(Parser::parse): Like in the other skeletons, issue the "starting
parse" message before any action.
Honor %initial-action.
Initialize the stacks with the lookahead.
* data/yacc.c: Let $$ and @$ in %initial-action designate the
look-ahead.
Push them in the stacks.
* tests/actions.at, tests/calc.at: Adjust the C++ ctor invocations.

tests/actions.at

@@ -327,7 +327,7 @@ static bool yydebug;
 int
 yyparse ()
 {
-  yy::Parser parser (yydebug, yy::Location ());
+  yy::Parser parser (yydebug);
   return parser.parse ();
 }
 ],

tests/calc.at

@@ -144,7 +144,7 @@ yy::Parser::error_ ()
 int
 yyparse (AT_PARAM_IF([semantic_value *result, int *count]))
 {
-  yy::Parser parser (!!YYDEBUG[]AT_LOCATION_IF([, yy::Location ()])AT_PARAM_IF([, result, count]));
+  yy::Parser parser (!!YYDEBUG[]AT_PARAM_IF([, result, count]));
   return parser.parse ();
 }
 ],