arcstat -p and fix for compatibility with py2/3

[justinpryzby]

System information

Type	Version/Name
Distribution Name	CentOS Linux
Distribution Version	7
Kernel Version	3.10.0-1160.11.1.el7
Architecture	x86_64
OpenZFS Version	2.0.6 and .7

Describe the problem you're observing

arcstat -p doesn't work with either version of python:

Describe how to reproduce the problem

$ /usr/bin/python3.6 /usr/bin/arcstat -p
time read miss miss% dmis dm% pmis pm% mmis mm% size c avail
Traceback (most recent call last):
File "/usr/bin/arcstat", line 521, in
main()
File "/usr/bin/arcstat", line 506, in main
print_values()
File "/usr/bin/arcstat", line 260, in print_values
sys.stdout.write(sep.join(fmt(col) for col in hdr))
TypeError: sequence item 1: expected str instance, float found

Also, arcstat without -p shows rounded values with only one significant figure when run with py2.7.
The rounding behavior changed in py3 (/ means FP division, and integer division is now //).

[justinpryzby]

-        num = num / scale
+        num = float(num) / scale
...
-        fmt = lambda col: v[col]
+        fmt = lambda col: str(v[col])