Flush stdout on finalizing of mysqldump/mysqlbinlog output
to avoid truncation.

The same patch has been applied to the mysqltest.cc code with
  commit 34ff714b
  Author: Magnus Svensson <msvensson@mysql.com>
  Date:   Fri Nov 14 11:06:56 2008 +0100
    WL#4189 Make mysqltest flush log file at close if logfile is stdout

but not to mysqldump.c/mysqlbinlog.cc