Remove Copy from PollFd

[JonathonReinhart]

What does this PR do

PollFd implementing Copy makes it easy to accidentally refer to the wrong object after putting one into an array.

This PR removes Copy from PollFd. This fixes #2630.

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

[JonathonReinhart]

I'll wait for a positive signal from maintainers that this is a good solution before I add a changelog entry.

[asomers]

This looks like a good approach to me. Could you please add a doctest that shows how to properly combine PollFd::new, poll, and PollFd::any (or PollFd::events or similar)? You could probably copy the current docs for PollFd::new and move them into the PollFd structure.

[JonathonReinhart]

Thanks @asomers. I believe This is ready for review and merge.

I'm not sure why tier3 (i686-unknown-hurd-gnu) is failing; it appears to be unrelated to this change.

[SteveLauC]

I'm not sure why tier3 (i686-unknown-hurd-gnu) is failing; it appears to be unrelated to this change.

Let me take a look at this

Update: Looks like libc 0.2.172 is broken on hurd, it is not related to this PR

Update: I will fix it in #2632

[SteveLauC]

Hi @JonathonReinhart, you can rebase your branch to fix the CI issues

[JonathonReinhart]

Looks like this is ready to be merged.