Cleanup: Integrate similar code to cgroup_get_uid_gid_from_procfs().

Cleanup: Integrate similar code to cgroup_get_uid_gid_from_procfs().

CHANGELOG of v2.1:
================
* Rebase the patch for commit '340feae163c4797a6cb1247b3812c1ccdc52fa41'.

There are some similar functions for getting process's data (uid, gid) from
/proc/<pid>/status file, so this patch integrates these functions into one
cgroup_get_uid_gid_from_procfs().

Signed-off-by: Ken'ichi Ohmichi <oomichi@mxs.nes.nec.co.jp>
Reviewed-By: Jan Safranek <jsafrane@redhat.com>
Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>