zpool iostat should print headers when terminal fills

When zpool iostat fills the terminal the headers should be
printed again. zpool iostat -n can be used to suppress this.

If the command is not attached to a tty, headers will not be
printed so as to not break existing scripts.

Reviewed-by: Joshua M. Clulow <josh@sysmgr.org>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Damian Wojsław <damian@wojslaw.pl>
Closes #8235
Closes #8262