Commit adds documentation for GetDiskSerialNumber(), because it wasn't clear
what was expected as a parameter.

Fixes #212

Old kernels have a bug in diskstats where lines can have less than 14
fields. This applies to the kernel present in RHEL 5.2 and earlier.

It's a bit of a niche but probably best to patch to be safe from future
bugs too.

RHEL bug case:
https://bugzilla.redhat.com/show_bug.cgi?id=583285

Encountered in Telegraf:
https://github.com/influxdata/telegraf/issues/1322

- Added conditional preprocessor guard on cpu_darwin_cgo.go

- Duplicated disk_darwin_amd64 for arm64 (after confirming
that sys/mount.h are the same between the two platforms, hence the
constants should be valid).

FreeBSD apparently changed the magic sysctl mib values for devstats.

    --- FAIL: TestDisk_io_counters (0.00s)
    disk_test.go:39: error no such file or directory
    disk_test.go:42: ret is empty, map[]

This code uses an undocumented, but exported, go stdlib method to fetch
the sysctl by string instead of mib.

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

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.

add used and usedPercent back in