This function used to be a private part of process.go.

Since I needed that functionality however I think it's better to make it public
than for me to copy it into my own code.

As a side effect of this change, I also fixed a bug in the function where Stolen
was not part of the sum. Having the function close to the CPUTimesStat
declaration will make problems like this less likely to re-occur in the future.

Fixes #104

Fixes #99

Package common wasn't used for public functions. Place it in an
internal directory to prevent other packages from using.

Remove the distributed references to "HOST_PROC" and "HOST_SYS"
consts and combine into a common function. This also helps so that
if a env var is defined with a trailing slash all will continue to
work as expected.

Fixes #100

ARM CPUs don't include the same fields as  x86 and amd64 CPUs in
the /proc/cpuinfo list. Pull information from the /sys/...
device tree as well as updating when a CPU is done in cpuinfo.

Fixes #88

Added the ability to fetch an alternative location for /proc via an
environment variable. If the env var is not set it will return /proc as
the default value.

Tested on windows 7:

When divided by 100 you get cpu percentage of  %0.14 when task manager shows %14. Once this is removed they match.

Note: This may not work on some old Windows XP.