refactor: make Epoll take BorrowedFd

[SteveLauC]

What does this PR do

1. Make Epoll-associated functions take BorrowedFd so that it is consistent with FdSet and PollFd
2. Update the tests in test_epoll.rs, use the new APIs

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

We cannot use _fds: std::marker::PhantomData<BorrowedFd<'fd>> here or the following code would compile without any issue:

use nix::sys::epoll::{Epoll, EpollCreateFlags, EpollEvent};
use nix::unistd::pipe;

fn main() {
    let (r, _w) = pipe().unwrap();
    let epoll = Epoll::new(EpollCreateFlags::empty()).unwrap();
    epoll.add(r, EpollEvent::empty()).unwrap();

    drop(r);
    drop(epoll);
}

I haven't figured out the reason, but using an invariant type here would work

[SteveLauC]

Friendly ping @asomers, do you have any idea about this?

[asomers]

Just to get rid of the unsafe, I suggest:

Suggested change

	let fd_1 = unsafe { BorrowedFd::borrow_raw(1) };
	let stdin = std::io::stdin();
	let fd_1 = stdin.as_fd();

[SteveLauC]

will do