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/symtab.h

@@ -51,11 +51,27 @@ typedef int symbol_number;
 
 typedef struct symbol symbol;
 
+/* Declaration status of a symbol.
+
+   First, it is "undeclared".  Then, if "undeclared" and used in a
+   %printer/%destructor, it is "used".  If not "declared" by used in a
+   rule, it is "needed".  Finally, if declared (via a rule for
+   nonterminals, or %oken), it is "declared".
+
+   When status are checked at the end, "declared" symbols are fine,
+   "used" symbols trigger warnings, otherwise it's an error.
+*/
+
 typedef enum
   {
-    needed,   /**< found but not "defined".  */
-    used,     /**< used by %printer but not declared.  */
-    declared, /**< defined with %type or %token.  */
+    /** Used in the input file for an unknown reason (error).  */
+    undeclared,
+    /** Used by %destructor/%printer but not defined (warning).  */
+    used,
+    /** Used in the gramar (rules) but not defined (error).  */
+    needed,
+    /** Defined with %type or %token (good).  */
+    declared,
   } status;
 
 /* When extending this structure, be sure to complete