Add arc_summary.py from FreeNAS

The arc_summary script is a useful utility for administrators on
other ZFS platforms.  It provides a quick and easy way to get a high
level view of the current ARC state.

Historically this was a perl script but it was rewritten in python
for FreeNAS.  We've decided to adopt the python version instead of
the perl version for a few reasons.

1) ZoL has no existing perl dependencies, but it does have a python
   dependency for scripts such as arcstat.py and dbufstat.py.  Using
   python for arc_summary.py helps us minimize dependencies.

2) Most major Linux distributions already depend heavily on python
   for their core infrastructure.  This means it's very likely to
   be available even very early in the boot process.

Original source:
  https://github.com/freenas/freenas/blob/master/gui/tools/arc_summary.py

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: cburroughs <chris.burroughs@gmail.com>
Signed-off-by: Kyle Blatter <kyleblatter@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>

cmd/arc_summary/Makefile.am

@@ -0,0 +1,2 @@
+bin_SCRIPTS = arc_summary.py
+EXTRA_DIST = $(bin_SCRIPTS)