Move get pid into cgroup implementation

This PR moves the logic to get the pids from the cgroup system into the cgroup implementations for systemd and raw filesystem.  

Tested on both raw filesystem and a systemd system with privileged and unprivileged containers. 

Closes #5964
Fixes #5666


Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)