use libcg to get cgroup mount point

[bgoglin]

An example get_mount_point.c is provided in the libcg source, it's visible in the Debian package git at https://salsa.debian.org/debian/libcgroup/-/blob/debian/master/samples/c/get_mount_point.c?ref_type=heads

Would be even easier than #678 but it is not clear whether libcg-devel is widely available, it doesn't have lots of reverse-dependencies.