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.

THANKS

@@ -44,6 +44,7 @@ Fabrice Bauzac            noon@cote-dazur.com
 Florian Krohm             florian@edamail.fishkill.ibm.com
 Frank Heckenbach          frank@g-n-u.de
 Frans Englich             frans.englich@telia.com
+Gabriel Rassoul           gabriel.rassoul@epita.fr
 Georg Sauthoff            gsauthof@TechFak.Uni-Bielefeld.DE
 George Neuner             gneuner2@comcast.net
 Gilles Espinasse          g.esp@free.fr