Use cgroup quotas for calculating `available_parallelism`

Use cgroup quotas for calculating `available_parallelism`

Manually tested via

```
// spawn a new cgroup scope for the current user
$ sudo systemd-run -p CPUQuota="300%" --uid=$(id -u) -tdS

// quota.rs
#![feature(available_parallelism)]
fn main() {
    println!("{:?}", std::thread::available_parallelism()); // prints Ok(3)
}
```

Caveats

* cgroup v1 is ignored
* funky mountpoints (containing spaces, newlines or control chars) for cgroupfs will not be handled correctly since that would require unescaping /proc/self/mountinfo
  The escaping behavior of procfs seems to be undocumented. systemd and docker default to `/sys/fs/cgroup` so it should be fine for most systems.
* quota will be ignored when `sched_getaffinity` doesn't work
* assumes procfs is mounted under `/proc` and cgroupfs mounted and readable somewhere in the directory tree