grammar: introduce %empty

Provide a means to explicitly denote empty right-hand sides of rules:
instead of

  exp:  { ... }

allow

  exp: %empty { ... }

Make sure that %empty is properly used.

With help from Joel E. Denny and Gabriel Rassoul.
http://lists.gnu.org/archive/html/bison-patches/2013-01/msg00142.html

* src/reader.h, src/reader.c (grammar_current_rule_empty_set): New.
* src/parse-gram.y (%empty): New token.
Use it.
* src/scan-gram.l (%empty): Scan it.
* src/reader.c (grammar_rule_check): Check that %empty is properly used.
* tests/actions.at (Invalid uses of %empty, Valid uses of %empty): New.

src/symlist.h

@@ -78,6 +78,10 @@ typedef struct symbol_list
    * each RHS are also stored here.  */
   code_props action_props;
 
+  /* The location of the first %empty for this rule, or \a
+     empty_location.  */
+  location percent_empty_loc;
+
   int dprec;
   int merger;
   location merger_declaration_location;