pgrep -P $PID exits with status of 1 (and nothing in stdout nor stderr) both if
a process doesn't exist or it doesn't have child processes, so we don't
use it anymore on these OSes. We sort PIDs as pgrep did.
Also deprecate the ErrorNoChildren error when there are no child processes,
this is erroneous (simply check for the length of the returned slice, plus
this is not an error per se), this was only returned on linux anyway.
Fixes #1698
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