feat: I/O safety for 'sys/signal'

[SteveLauC]

What does this PR do

1. Adds I/O safety for sys/signal.

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

[asomers]

Apart from the doc nit, this LGTM. However, it may cause borrow checker issues upstack. The change works for me in mio-aio, but it could cause difficulty if Tokio adopts I/O Safety. I think we should probably merge it, but work with Tokio to ensure that all of the pieces will fit together before we release.

[SteveLauC]

macOS CI failure: network issue

    Updating crates.io index
warning: spurious network error (2 tries remaining): SecureTransport error: (null); class=Net (12)
warning: spurious network error (1 tries remaining): SecureTransport error: (null); class=Net (12)
error: failed to get `bitflags` as a dependency of package `nix v0.26.1 (/private/var/folders/hy/gdmd645x1ws410vx266t7py00000gn/T/cirrus-ci-build)`

Caused by:
  failed to load source for dependency `bitflags`

Caused by:
  Unable to update registry `crates-io`

Caused by:
  failed to fetch `https://github.com/rust-lang/crates.io-index`

Caused by:
  network failure seems to have happened
  if a proxy or similar is necessary `net.git-fetch-with-cli` may help here
  https://doc.rust-lang.org/cargo/reference/config.html#netgit-fetch-with-cli

Caused by:
  SecureTransport error: (null); class=Net (12)

Exit status: 101

[SteveLauC]

but it could cause difficulty if Tokio adopts I/O Safety.

Hi @asomers, I think we can merge this now, looks like Tokio won't adopt I/O safety.

[SteveLauC]

I changed the associated type of this variant to &'fd Infallible, since Infallible is an enum with 0 variants, this variant can now never be constructed. (As a contrast, PhantomData is very easy to create)

[SteveLauC]

Remove this header, looks like it is the only header we have.

[asomers]

I just checked again. There's no way to make this work with Tokio except by using BorrowedFd::borrow_raw, which defeats the whole point of I/O safety. But we can merge it if there are other users who can benefit.

[SteveLauC]

There's no way to make this work with Tokio except by using BorrowedFd::borrow_raw

Ah, right, since Tokio does not adopt I/O safety (in its implementation), all its internal interfaces are using RawFd, so it has to do the tedious conversion when interacting with I/O-safe interfaces.