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.

examples/c/lexcalc/parse.y

@@ -76,10 +76,10 @@
 ;
 
 %token <int> NUM "number"
-%type <int> exp
+%type <int> exp expression line
 %printer { fprintf (yyo, "%d", $$); } <int>
 
-%start input line
+%start input expression
 
 // Precedence (from lowest to highest) and associativity.
 %left "+" "-"
@@ -93,8 +93,12 @@ input:
 ;
 
 line:
-  exp EOL   { printf ("%d\n", $exp); }
-| error EOL { yyerrok; }
+  exp EOL   { $$ = $exp; printf ("%d\n", $$); }
+| error EOL { $$ = 0; yyerrok; }
+;
+
+expression:
+  exp EOL  { $$ = $exp; }
 ;
 
 exp:
@@ -129,16 +133,22 @@ int main (int argc, const char *argv[])
   int nerrs = 0;
   // Possibly enable parser runtime debugging.
   yydebug = !!getenv ("YYDEBUG");
+  int parse_expression_p = 0;
   // Enable parse traces on option -p.
-  int parse_line_p = 0;
   for (int i = 0; i < argc; ++i)
     if (1 < argc && strcmp (argv[1], "-p") == 0)
       yydebug = 1;
-    else if (strcmp (argv[i], "-l") == 0)
-      parse_line_p = 1;
+    else if (strcmp (argv[i], "-e") == 0)
+      parse_expression_p = 1;
 
-  if (parse_line_p)
-    yyparse_line (&nerrs);
+  if (parse_expression_p)
+    {
+      yyparse_expression_t res = yyparse_expression (&nerrs);
+      if (res.yystatus == 0)
+        printf ("expression: %d\n", res.yyvalue);
+      else
+        printf ("expression: failure\n");
+    }
   else
     yyparse_input (&nerrs);
   // Exit on failure if there were errors.