Merge #1874

1874: signalfd optional file descriptor r=asomers a=JonathanWoollett-Light

[`sys::signalfd::signalfd`](https://docs.rs/nix/latest/nix/sys/signalfd/fn.signalfd.html) currently takes a `RawFd` for its `fd` argument.

Considering from [the documentation](https://man7.org/linux/man-pages/man2/signalfd.2.html):
> If the fd argument is -1, then the call creates a new file
       descriptor and associates the signal set specified in mask with
       that file descriptor.  If fd is not -1, then it must specify a
       valid existing signalfd file descriptor, and mask is used to
       replace the signal set associated with that file descriptor.

We can better pass the argument as `Option<BorrowedFd>` which encodes the optional nature of this parameter in an option rather than the value being -1 (invalid) (`size_of::<Option<BorrowedFd>>() == size_of::<RawFd>() == 4`).

This removes the error case where `fd < -1`.

> EBADF  The fd file descriptor is not a valid file descriptor.

This does however require additional changes to produce a cohesive implementation, notably changing the type within `Signal` from `RawFd` to `ManuallyDrop<OwnedFd>`, this has no functional affect, but illustrates ownership and allows the type to more easily produce `BorrowedFd`s.

To use [`BorrowedFd`](https://doc.rust-lang.org/stable/std/os/unix/io/struct.BorrowedFd.html) requires updating the MSRV to `>= 1.63.0`

Co-authored-by: Jonathan <jonathanwoollettlight@gmail.com>

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