process: add support for cgroups

Summary: Signed-off-by: Topi Miettinen <toiwoton@gmail.com>

Reviewers: Plasma, hein, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20787