refactor: make Epoll take BorrowedFd

[SteveLauC]

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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:

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