core: add SmackIpIn/SmackIpOut key for sockets

This adds SMACK label configuration options to socket units.

Sockets can be created at runtime and uselessd does just
that for several services. In order to protect UNIX domain
sockets, we must instruct uselessd to apply SMACK labels at
runtime.

This patch adds the following options :

SmackIpIn/SmackIpOut - applicable to sockets

Change-Id: Ia0d884d2a5f119f9d09275ab6a0fa4afcdfce9eb
Origin: systemd@0eb59ccfe619cbc4b42ef8ff02b52971994dfe05
Author: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>