multistart: use b4_accept instead of action post-processing

For each start symbol, generate a parsing function with a richer
return value than the usual of yyparse.  Reserve a place for the
returned semantic value, in order to avoid having to pass a pointer as
argument to "return" that value.  This also makes the call to the
parsing function independent of whether a given start-symbol is typed.

For instance, if the grammar file contains:

    %type <int> expression
    %start input expression

(so "input" is valueless) we get

    typedef struct
    {
      int yystatus;
    } yyparse_input_t;

    yyparse_input_t yyparse_input (void);

    typedef struct
    {
      int yyvalue;
      int yystatus;
    } yyparse_expression_t;

    yyparse_expression_t yyparse_expression (void);

This commit also changes the implementation of the parser termination:
when there are multiple start symbols, it is the initial rules that
explicitly YYACCEPT.  They do that after having exported the
start-symbol's value (if it is typed):

  switch (yyn)
    {
  case 1: /* $accept: YY_EXPRESSION expression $end  */
  { ((*yyvalue).TOK_expression) = (yyvsp[-1].TOK_expression); YYACCEPT; }
    break;

  case 2: /* $accept: YY_INPUT input $end  */
  { YYACCEPT; }
    break;

I have tried several ways to deal with termination, and this is the
one that appears the best one to me.  It is also the most natural.

* src/scan-code.h, src/scan-code.l (obstack_for_actions): New.
* src/reader.c (grammar_rule_check_and_complete): Generate the actions
of the rules for each start symbol.

* data/skeletons/bison.m4 (b4_symbol_slot): New, with safer semantics
than type and type_tag.
* data/skeletons/yacc.c (b4_accept): New.
Generates the body of the action of the start rules.
(_b4_declare_sub_yyparse): For each start symbol define a dedicated
return type for its parsing function.
Adjust the declaration of its parsing function.
(_b4_define_sub_yyparse): Adjust the definition of the function.

* examples/c/lexcalc/parse.y: Check the case of valueless symbols.
* examples/c/lexcalc/lexcalc.test: Check start symbols.

src/scan-code.l

@@ -40,6 +40,8 @@
 #undef code_wrap
 #define code_wrap() 1
 
+struct obstack *obstack_for_actions = &obstack_for_string;
+
 /* The current calling start condition: SC_RULE_ACTION or
    SC_SYMBOL_ACTION. */
 # define YY_DECL static char *code_lex (code_props *self, int sc_context)
@@ -756,19 +758,10 @@ handle_action_at (symbol_list *rule, char *text, const location *at_loc)
 static char const *
 translate_action (code_props *self, int sc_context)
 {
-  static bool initialized = false;
-  if (!initialized)
-    {
-      obstack_init (&obstack_for_string);
-      yy_flex_debug = 0;
-      initialized = true;
-    }
-
   loc->start = loc->end = self->location.start;
   yy_switch_to_buffer (yy_scan_string (self->code));
   char *res = code_lex (self, sc_context);
   yy_delete_buffer (YY_CURRENT_BUFFER);
-
   return res;
 }
 
@@ -845,6 +838,13 @@ code_scanner_last_string_free (void)
   STRING_FREE ();
 }
 
+void
+code_scanner_init (void)
+{
+  obstack_init (&obstack_for_string);
+  yy_flex_debug = 0;
+}
+
 void
 code_scanner_free (void)
 {