pkg/cgroups Add GetMounts() and GetSubsystems()

This lists all currently mounted cgroups and all supported cgroup
subsystems on the machine.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
libcontainer: Mount cgroups in the container

We make a tmpfs on /sys/fs/cgroups, and here we mount read-only
versions of all the host cgroups. Additionally we make symlinks
for all merged subsystems.

For any "named" cgroup, such as "name=systemd" we also mount the
subset of the cgroup where the container lives as read-write. This
means that the container can create sub-cgroups inside the container
and move tasks into those, but it can never escape from its current
position in the cgroup hierarchy.

In particular, this allows systemd to mostly work in a non-privileged
container. The only problem currently is that PrivateTmp=true fails
because systemd is not allowed to mount a new /tmp.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)