Merge branch 'arc_summary_draft_v2'

Add a port of arc_summary.py to ZFS on Linux, arc_summary.py is a
standard tool in FreeBSD and Illumos. The version of the script used
for the port originally came from FreeNAS.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kyle Blatter <kyleblatter@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes #2920