style: s/non-terminal/nonterminal/

I personally prefer 'non terminal', or 'non-terminal', but
'nonterminal' is the common spelling.

* data/glr.c, src/parse-gram.y, src/symtab.c, src/symtab.h,
* tests/input.at, doc/refcard.tex: here.

src/symtab.h

@@ -40,7 +40,7 @@ typedef enum
 {
   unknown_sym,          /**< Undefined.  */
   token_sym,            /**< Terminal. */
-  nterm_sym             /**< Non-terminal. */
+  nterm_sym             /**< Nonterminal. */
 } symbol_class;