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
Add common.invoker interface to mock exec.Command. common.FakeInvoker returns
expected file if exists instead of invoke exec.Command.
Currenly, mocking is enabled only process.Pids(). I will expand to other funcs incrementally.
