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.c

@@ -44,6 +44,7 @@ symbol_list_sym_new (symbol *sym, location loc)
 
   /* Members used for LHS only.  */
   res->ruleprec = NULL;
+  res->percent_empty_loc = empty_location;
   code_props_none_init (&res->action_props);
   res->dprec = 0;
   res->merger = 0;