Improve the CPU core count detection

See theofidry/cpu-core-counter#11 for the motivations.

The code should be sensibly the same, the notable differences are:

- fixed the deprecated usage of `hw.ncpu`
- /proc/cpuinfo is check _last_
- nproc is checked first (before was not checked at all, it's considered to be more accurate and less convoluted than cpuinfo though)
- not sure about the `return 2`, but this was not too clear [here](phpstan#514 (review)) neither. I could otherwise change the fallback value to return `1` if `proc_open` doesn't exist and 2 otherwise
- add more ways to find the CPU cores count