diagnostics: replace "user token number" by "token code"

Yet, don't change the structure identifier to avoid introducing
conflicts in Vincent Imbimbo's PR (which, amusingly enough, is about
conflicts).

* src/symtab.c: here.
* tests/diagnostics.at, tests/input.at: Adjust.

TODO

@@ -5,6 +5,7 @@
 - YYERRCODE, YYUNDEF, YYEOF
 - i18n in Java
 - symbol.type_get should be kind_get, and it's not documented.
+- token code instead of token user number etc.
 
 ** User token number, internal symbol number, external token number, etc.
 There is some confusion over these terms, which is even a problem for
@@ -28,21 +29,6 @@ The token numbered as 0.
 Therefore each time the scanner returns an (external) token number,
 it must be mapped to the (internal) symbol number.
 
-
-*** The code
-uses "user token number" in most places.
-
-  if (sym->content->class != token_sym)
-    complain (&loc, complaint,
-              _("nonterminals cannot be given an explicit number"));
-  else if (*user_token_numberp != USER_NUMBER_UNDEFINED
-           && *user_token_numberp != user_token_number)
-    complain (&loc, complaint, _("redefining user token number of %s"),
-              sym->tag);
-  else if (user_token_number == INT_MAX)
-    complain (&loc, complaint, _("user token number of %s too large"),
-              sym->tag);
-
 *** M4
 Make it consistent with the rest (it uses "user_number" and "number").