Minor code cleanup in arc_summary.py

Minor code cleanup in arc_summary.py

Simplify and inline single-use function div1(); inline twice-used
function div2(); add function comment to zfs_header(); replace
variable "unused" in get_Kstat() with "_" following convention.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Scot W. Stevenson <scot.stevenson@gmail.com>
Closes #6802