Add Python 3 rewrite of arc_summary.py (openzfs#6873)

Add new script arc_summary3.py as a complete rewrite of the
arc_summary.py tool (see issue openzfs#6873). Add test_arc_summary3.py
as (incomplete) unittest test suite.

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.
Adds first (trivial) tests. Note the associated test file
currently opts out of flake8 testing.

Developed for Python 3.5.

Signed-off-by: Scot W. Stevenson <scot.stevenson@gmail.com>

cmd/arc_summary/Makefile.am

@@ -1 +1,2 @@
 dist_bin_SCRIPTS = arc_summary.py
+dist_bin_SCRIPTS = arc_summary3.py