printer/destructor: translate only once

Currently "%printer {...} a b c d e f" translates the {...} six times.
Not only is this bad for time and space, it also issues six times the
same warnings.

* src/symlist.h, src/symlist.c (symbol_list_destructor_set)
(symbol_list_printer_set): Take the action as code_props instead of
const char *.
* src/parse-gram.y: Translate these actions here.
* src/scan-code.h: Comment change.
* tests/input.at: Check that warnings are issued only once.

src/scan-code.h

@@ -45,7 +45,10 @@ typedef struct code_props {
     CODE_PROPS_SYMBOL_ACTION, CODE_PROPS_RULE_ACTION
   } kind;
 
-  /** \c NULL iff \c code_props::kind is \c CODE_PROPS_NONE.  */
+  /**
+   * \c NULL iff \c code_props::kind is \c CODE_PROPS_NONE.
+   * Memory is allocated in an obstack freed elsewhere.
+   */
   char const *code;
   /** Undefined iff \c code_props::code is \c NULL.  */
   location location;