cgroup-util: before operating on a mounted cgroup controller check if it actually can be mounted

We now have the "BPF" pseudo-controllers. These should never be assumed
to be accessible as /sys/fs/cgroup/<controller> and not through
"cgroup.subtree_control" either, hence always check explicitly before we
go to the file system. We do this through our new CGROUP_MASK_V1 and
CGROUP_MASK_V2 definitions.