tokens: define the "$undefined" token kind

* data/skeletons/bison.m4 (b4_symbol_token_kind): Give a definition to
$undefined.
(b4_token_visible_if): $undefined has an id.
* src/output.c (prepare_symbol_definitions): Stop lying: $undefined
_is_ a token.
* tests/input.at: Adjust.

src/output.c

@@ -549,8 +549,7 @@ prepare_symbol_definitions (void)
       MUSCLE_INSERT_INT (key, sym->content->user_token_number);
 
       SET_KEY ("is_token");
-      MUSCLE_INSERT_INT (key,
-                         i < ntokens && sym != undeftoken);
+      MUSCLE_INSERT_INT (key, i < ntokens);
 
       SET_KEY ("number");
       MUSCLE_INSERT_INT (key, sym->content->number);