cgroups: Add more systemd unit --opt options to control cgroups

This makes the existing unit.slice property work, and adds options
for the normal cgroup settings for cpu.*, cpuset.*, memory.*.

The cpu and memory ones use systemd to set up the cgroups, and then
manually tweak the properties from there. The cpuset cgroup is not
supported in systemd, and is a bit weird, so we do that ourself.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)