Add python style checking

Introduce a make recipe for flake8 to enable python
style checking. Ensure all python scripts pass flake8.
Return an error code of 0 for arcstat.py -v and
dbufstat.py -v.  Add test cases for python scripts.

Reviewed by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ian Lee <IanLee1521@gmail.com>
Closes #5230

diff --cc tests/runfiles/linux.run
Simple merge