Change the interface of `openat(dirfd: RawFd, ...)` to `openat(dirfd: Option<RawFd>, ...)`

[SteveLauC]

Almost every xxxat() function takes an optional dirfd, except fcntl::openat(2) and Dir::openat()

pub fn openat<P: ?Sized + NixPath>(
    dirfd: RawFd,
    path: &P,
    oflag: OFlag,
    mode: Mode,
) -> Result<RawFd> 

pub fn openat<P: ?Sized + NixPath>(dirfd: RawFd, path: &P, oflag: OFlag, mode: sys::stat::Mode) -> Result<Self> {
    let fd = fcntl::openat(dirfd, path, oflag, mode)?;
    Dir::from_fd(fd)
}

Any reason why it does not take an Option<RawFd>?

[SUPERCILEX]

We should just do something similar to https://docs.rs/rustix/latest/src/rustix/fs/cwd.rs.html#26-32 (but as a const) while implementing #1750.

[asomers]

Git archaeology doesn't show any good reason why the functions take a RawFd. I agree that an Option would be better.

[SteveLauC]

Close as completed in #2139