tokens: properly define the "error" token kind

There are people out there that do use YYERRCODE (the token kind of
the error token).  See for instance
https://github.com/borbolla-automation/SPC_Machines/blob/3812012bb782bfdfe7b325950a35cd337925fcad/unixODBC-2.3.2/Drivers/nn/yylex.c.

Currently, YYERRCODE is defined by yacc.c in an adhoc way as a #define
in the *.c file only.  It belongs with the other token kinds.

YYERRCODE is not a nice name, it does not fit in our naming scheme.
YYERROR would be more logical, but it collides with the YYERROR macro.
Shall we keep the same name in all the skeletons?  Besides, to avoid
collisions in C, we need to apply the api prefix: YYERRCODE is
actually <PREFIX>ERRCODE.  This is not needed in the other languages.

* data/skeletons/bison.m4 (b4_symbol_token_kind): New.
Map the error token to "YYERRCODE".
* data/skeletons/yacc.c (YYERRCODE): Don't define it, it's handled by...
* src/output.c (prepare_symbol_definitions): this.
* tests/input.at (Redefining the error token): Check it.

src/output.c

@@ -550,7 +550,7 @@ prepare_symbol_definitions (void)
 
       SET_KEY ("is_token");
       MUSCLE_INSERT_INT (key,
-                         i < ntokens && sym != errtoken && sym != undeftoken);
+                         i < ntokens && sym != undeftoken);
 
       SET_KEY ("number");
       MUSCLE_INSERT_INT (key, sym->content->number);

src/parse-gram.c

@@ -927,9 +927,6 @@ enum { YYENOMEM = -2 };
       }                                                           \
   while (0)
 
-/* Error token external number. */
-#define YYERRCODE       256
-
 
 /* YYLLOC_DEFAULT -- Set CURRENT to span from RHS[1] to RHS[N].
    If N is 0, then set CURRENT to the empty location which ends

src/parse-gram.h

@@ -78,6 +78,7 @@ extern int gram_debug;
   enum gram_tokentype
   {
     GRAM_EOF = 0,                  /* "end of file"  */
+    GRAM_ERRCODE = 1,              /* error  */
     STRING = 3,                    /* "string"  */
     TSTRING = 4,                   /* "translatable string"  */
     PERCENT_TOKEN = 5,             /* "%token"  */