Added column definitions to arcstat.py

grow: ARC Grow enabled (!arc_no_grow)
free: ARC Free memory (arc_sys_free)
need: ARC Reclaim need (arc_need_free)

Fixed alignment issues (mread had wrong width).

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gregor Kopka <gregor@kopka.net>
Closes #8058