Auto merge of rust-lang#3712 - tiif:feat/epoll, r=oli-obk

Implement epoll shim

This PR:
- implemented non-blocking ``epoll`` for rust-lang#3448 . The design for this PR is documented in https://hackmd.io/`@tiif/SJatftrH0` .
- renamed FileDescriptor to FileDescriptionRef
- assigned an ID to every file description

Author: bors