This commit adds extra setup when cgroupsv2 is enabled. In particular,
we make sure that the root namespace has setup cgroup.subtree_control
with the controllers we need.
If the necessary controller are not listed, we have to move all
processes out of the root namespace before we can change this
(the 'no internal processes' rule:
https://unix.stackexchange.com/a/713343). Currently we only
handle the case where the nsjail process is the only process in
the cgroup. It seems like this would be relatively rare, but since
nsjail is frequently the root process in a Docker container (e.g.
for hosting CTF challenges), I think this case is common enough to
make it worth implementing.
This also adds `--detect_cgroupv2`, which will attempt to detect
whether `--cgroupv2_mount` is a valid cgroupv2 mount, and if so
it will set `use_cgroupv2`. This is useful in containerized
environments where you may not know the kernel version ahead of time.
References:
https://github.com/redpwn/jail/blob/master/internal/cgroup/cgroup2.go
The `writeToFd` function in `util.cc` returns `ssize_t` but the only
returned values are either `false` or `true`.
```
ssize_t writeToFd(int fd, const void* buf, size_t len) {
(...) return false;
(...) return true;
```
