core: initialize Smack at startup, make it configurable

Smack can now be enabled at configure time (contrary to
systemd, it is disabled by default).

If enabled, at startup, uselessd will try to read Smack
default rules from their predefined path, and apply them
by writing to the kernel driver file descriptor.

Change-Id: Ifdc5a35302246c3f638a292da8b87b02abb93a9e
Origin: systemd@ffbd2c4d45787ba5ba85a32db6551efba66a1ee6
Origin: systemd@fea7838e7e0b2724f5e0bc028121a08b42995045
Origin: systemd@2b3e18de74ca89b374dd4f7a2c30e5731d347841
Author: Nathaniel Chen <nathaniel.chen@intel.com>
Author: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Author: Karol Lewandowski <k.lewandowsk@samsung.com>
Author: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>
Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>