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/scan-gram.l

@@ -217,6 +217,7 @@ eqopt    ([[:space:]]*=)?
   "%defines"                        return PERCENT_DEFINES;
   "%destructor"                     return PERCENT_DESTRUCTOR;
   "%dprec"                          return PERCENT_DPREC;
+  "%empty"                          return PERCENT_EMPTY;
   "%error-verbose"                  return PERCENT_ERROR_VERBOSE;
   "%expect"                         return PERCENT_EXPECT;
   "%expect-rr"                      return PERCENT_EXPECT_RR;