firewalls are remote, and this command will kill the network connection
to them), prompt the user for confirmation of this command.
Also, add the '-f' flag which ignores the need for confirmation the
command, and if there is no controlling tty (isatty(STDIN_FILENO) !=0)
assume '-f'.
If anyone is using ipfw flush in scripts it shouldn't affect them, but you
may want to change the script to use a 'ipfw -f flush'.
Reviewed by: alex
1285c95c4b