Use YYFPRINTF instead of fprintf where appropriate. Reported by

Sébastien Fricker at
<http://lists.gnu.org/archive/html/bug-bison/2007-02/msg00035.html>.
* THANKS: Add Sébastien Fricker.
* data/glr.c, data/push.c, data/yacc.c (yy_reduce_print): Implement.
* doc/bison.texinfo (Tracing): Make it clearer that YYFPRINTF must
accept a variable number of arguments.