%nterm: do not accept character literals

Reported by Rici Lake.
http://lists.gnu.org/archive/html/bug-bison/2018-10/msg00000.html

* src/complain.h: Formatting change.
* src/parse-gram.y (id): Reject character literals used in a context
for non-terminals.
* tests/input.at (Invalid %nterm uses): Check that.

src/complain.h

@@ -78,8 +78,7 @@ void complain_init (void);
 
 typedef enum
   {
-    /**< Issue no warnings.  */
-    Wnone             = 0,
+    Wnone             = 0,       /**< Issue no warnings.  */
 
     Wmidrule_values   = 1 << warning_midrule_values,
     Wyacc             = 1 << warning_yacc,