core: run with a custom Smack domain (label)

Allows the uselessd --system process to change its current
SMACK label to a predefined custom label (usually "System")
at boot time.

This is needed to have a few system-generated folders and
sockets automatically be created with the right SMACK
label, and for uselessd itself to be able to read and write
privilegied folders. Without that, processes either cannot
communicate with uselessd or uselessd fails to perform some
actions.

Change-Id: Ib78857392e963f39360192001afbe68eb0ee89cf
Origin: systemd@8b197c3a8a57c3f7c231b39e5660856fd9580c80
Author: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>