c++: report the stack at the same places as in C

Let's have C be the reference, and match it elsewhere.  Maybe C is too
verbose and some adjustments are needed, but then that would be done
in another batch of patches.

* data/skeletons/lalr1.cc: Print the stack once we popped after
YYERROR, and before emptying the stack at the end of parsing.

tests/c++.at

@@ -679,6 +679,7 @@ Entering state 2
 Reading a token: Next token is token EOI ()
 Shifting token EOI ()
 Entering state 3
+Stack now 0 2 3
 Cleanup: popping token EOI ()
 Cleanup: popping nterm expr (40)
 destroy: 40