yacc: fix YYBACKUP.

Reported by David Kastrup:
https://lists.gnu.org/archive/html/bug-bison/2011-10/msg00002.html.

	* data/yacc.c (YYBACKUP): Accept rhs size.
	Restore the proper state value.
	* TODO (YYBACKUP): Make it...
	* tests/actions.at: a new test case.
	* NEWS, THANKS: Update.
(cherry picked from commit d115aad9112fb4e2fe1b268c9db7390732d39539)

Conflicts:

	TODO
	data/yacc.c