execute: add SmackExecLabel key for services

This commit introduces a new "SmackExecLabel" key for unit
files, so we can set the runtime Smack label for their
executables. For example, 'uselessd --user' may need to run
with a different label as compared to 'uselessd --system',
so "SmackExecLabel" could be set in user@.service.

Setting the label will fail if the service lacks
capabilities to set its own Smack label.

Change-Id: I4f5c908825585106e31c1132e9bb64d30f28349d
Origin: tizen@5b3ac1d8e322c32f96271d09c3ed09bc30ee4db3
Author: Patrick McCarty <patrick.mccarty@linux.intel.com>
Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>