Import Linux cgroup support

[nobu]

From https://github.com/moznion/maxprocs-ruby

[Feature #21797]

[moznion]

Thank you for incorporating the functionality of my library.
I have one quick question: when a user calls nprocessors, does it read the file every time? Is that acceptable?

[nobu]

I think it is same as that Etc.nprocessors makes a system call every time it is called.
And usually it won't be called so many times in a process.

[moznion]

Thank you, I understand that.