fix warnings for useless %printer/%destructor

The previous commit, which turns into a warning what used to be an
error:

    %printer {} foo;
    %%
    exp: '0';

has two shortcomings: the warning is way too long (foo is reported
to be useless later), and besides, it also turns into a warning much
more serious errors:

   %printer {} foo;
   %%
   exp: foo;

Reduce the amount to warnings in the first case, restore the error in
the second.

* src/symtab.h (status): Add a new inital state: undeclared.
* src/symtab.c (symbol_new): Initialize to undeclared.
(symbol_class_set): Simplify the logic of the code that neutralize
the "redeclared" warning after the "redefined" one.
(symbol_check_defined): "undeclared" is also an error.
* src/reader.c (grammar_current_rule_symbol_append): Symbols appearing
in a rule are "needed".
* src/symlist.c (symbol_list_destructor_set, symbol_list_printer_set):
An unknown symbol appearing in a %printer/%destructor is "used".
* src/reduce.c (nonterminals_reduce): Do not report as "useless" symbols
that are not used (e.g., those that for instance appeared only in a
%printer).
* tests/input.at (Undeclared symbols used for a printer or destructor):
Improve the cover the cases described above.

src/reduce.c

@@ -298,8 +298,10 @@ nonterminals_reduce (void)
     if (!bitset_test (V, i))
       {
         nontermmap[i - ntokens] = n++;
-        warn_at (symbols[i]->location, _("nonterminal useless in grammar: %s"),
-                 symbols[i]->tag);
+        if (symbols[i]->status != used)
+          warn_at (symbols[i]->location,
+                   _("nonterminal useless in grammar: %s"),
+                   symbols[i]->tag);
       }