Add Python 3 rewrite of arc_summary.py

Add new script arc_summary3.py as a complete rewrite of the
arc_summary.py tool (see issue #6873)

Add new options:

        -g/--graph    - Display crude graphic representation
                        of ARC status and quit
        -r/--raw      - Print all available information as
                        minimally formatted list (for grep)
        -s/--section  - Print a single section. This
                        replaces -p/--page, which is kept for
                        backwards use but marked as
                        depreciated

Add new sections with information on ZIL and SPL. Notify user
if sections L2ARC and VDEV are skipped instead of failing
silently. Add warning that -p/--page option is depreciated.

Developed for Python 3.5.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Scot W. Stevenson <scot.stevenson@gmail.com>
Closes #6873 
Closes #6892

tests/zfs-tests/include/commands.cfg

@@ -91,6 +91,7 @@ export SYSTEM_FILES='arp
     ps
     pwd
     python
+    python3
     quotaon
     readlink
     rm
@@ -143,6 +144,7 @@ export ZFS_FILES='zdb
     ztest
     raidz_test
     arc_summary.py
+    arc_summary3.py
     arcstat.py
     dbufstat.py
     zed