Add arcstat.py from freenas

arcstat.py is a python port of arcstat.pl that's included in freenas.  It appears to be a line by line port.  It requires read permission only in the /proc filesystem, so it can be run as a normal user.

The only modified function is def kstat_update(): with some minor header changes to remove an unneeded import.  Tested and works as intended.  All the arcstat functionality is in tact.

Key advantages over arcstat.pl:
1) doesn't require kstat.pm

Other key issues:
1) Most distros include python nowadays.  It should work on all of them by default.
2) Doesn't conflict with arcstat.pl because they've got different extensions.