pidwait: `waitpid` via `epoll()`

[FurryR]

Currently the implementation waitpid is using spin waiting, which could lead to performance issues. It is possible to refactor it via pidfd_open and epoll to allow CPU to save battery.

This issue is used for being linked to any future Pull Requests.

procps/src/uu/pidwait/src/wait.rs

Line 28 in 1194b3f

sleep(Duration::from_millis(50));

[FurryR]

Linked a possible solution here. The new implementation referred to waitpid-any, which is distributed under either MIT License or Apache-2.0 License, so it should be fine. I also credited the author in the source file.