cgroup: use an empty mount list on error

If the proc filesystem does not exist, return an empty list:

    Port = alcove_drv:start([{exec, "sudo"}]),
    Child = alcove:fork(Port),
    true = alcove_cgroup:supported(Port, [Child]),
    ok = alcove:chroot(Port, [Child], "/bin"),
    ok = alcove:chdir(Port, [Child], "/"),
    false = alcove_cgroup:supported(Port, [Child]).