Fix problem reported by Henrik Grubbstroem in

 Fix problem reported by Henrik Grubbstroem in
 <http://mail.gnu.org/pipermail/bug-bison/2002-October/001670.html>:
 "nonterm: { $$ = 123; } { $$ = $1; };" was wrongly rejected,
 because the Bison parser reads the second action before reducing
 the first one.
 * src/scan-gram.l (rule_length): New static var.
 Use it to keep track of the rule length in the scanner, since
 we can't expect the parser to be in lock-step sync with the scanner.
 (handle_action_dollar, handle_action_at): Use this var.
 * tests/actions.at (Exotic Dollars): Test for the problem.