#41869 added cgroup v2 support for contrib/check-config.sh, but it doesn't verify available controllers currently.

We need to verify that

• availability of cpu, cpuset, memory, and io controllers, by reading /sys/fs/cgroup/cgroup.controllers file
• availability of BPF_CGROUP_DEVICE support, probably by checking that kernel >= 4.15 && CONFIG_CGROUP_BPF. This is an essential requirement for rootful mode.
• availability of freezer, probably by checking the existence of cgroup.freezer file. This requires kernel >= 5.2 and is optional.