libksysguard: show Mandatory Access Control (SELinux/AppArmor) context

Summary:
Collect and display Mandatory Access Control (SELinux/AppArmor) context info from /proc/PID/attr/current.

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

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20854