- list all cgroup v1 mountpoints
- list all cgroup v2 mountpoints
- report "missing" when no mountpoint for the systemd controller was found
- report "missing" when no mountpoint for the freezer controller was found

Closes https://github.com/lxc/lxd/issues/3687.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>