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