Modified arc_summary.py to run on linux

1. Comment out stat sections whose kstats are not currently available
2. Port most of arc_summary to use spl kstats
3. Enable l2arc stats
4. Include compressed l2size
5. Minor style fixes / cleanup

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>