AppArmor: Support for loading a set of pre-compiled profiles at startup time

AppArmor: Support for loading a set of pre-compiled profiles at startup time

Let systemd load a set of pre-compiled AppArmor profile files from a policy
cache at /etc/apparmor/earlypolicy. Maintenance of that policy cache must be
done outside of systemd.

After successfully loading the profiles systemd will attempt to change to a
profile named systemd.

If systemd is already confined in a profile, it will not load any profile files
and will not attempt to change it's profile.

If anything goes wrong, systemd will only log failures. It will not fail to
start.