Allow cgroup handler to access /sys/fs/cgroup as tmpfs_t

Currently, the /sys/fs/cgroup location is mounted as a tmpfs_t. As the
mount options cannot be easily modified as of yet, we grant the cgroup
handler search privileges over tmpfs_t.

Additional cgroup mounts within /sys/fs/cgroup do hold the right context
(cgroup_t).
Fix bug 530898 - Enable netlink interaction from dhcpcd