feat: AT_EMPTY_PATH for fchownat on Linux/Android/FreeBSD/Hurd

[SteveLauC]

What does this PR do

Picks up #1128

Closes #1128 Closes #1029

Enable the AT_EMPTY_PATH flag for fchownat() on Linux/Android/FreeBSD/Hurd

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

[SteveLauC]

For this type, I am not sure if we should define a new type or just use the AtFlags type.

It is safer to define a new type so that we can prevent users from passing invalid arguments, but this will add our maintainence burgen. We are already using the AtFlags type for other functions like fstatat

[asomers]

If we're already using AtFlags for more than one function, I think we can use it here, too.

[SteveLauC]

Get it

[asomers]

This is a rare case of an API change in Nix where we can help users migrate. What about defining some deprecated constants like this?

impl FchownatFlags {
     #[deprecated]
    pub const FollowSymlink = FchownatFlags::empty();
    #[deprecated]
    pub const NoFollowSymlink = FchownatFlags::AT_SYMLINK_NOFOLLOW;
}

[asomers]

If we're already using AtFlags for more than one function, I think we can use it here, too.

[asomers]

It's also available on FreeBSD, beginning in 13.1.

[SteveLauC]

Let me do this in a separate PR

[SteveLauC]

This is a rare case of an API change in Nix where we can help users migrate. What about defining some deprecated constants like this?

impl FchownatFlags {
     #[deprecated]
    pub const FollowSymlink = FchownatFlags::empty();
    #[deprecated]
    pub const NoFollowSymlink = FchownatFlags::AT_SYMLINK_NOFOLLOW;
}

Done

---

Well, fixing imports is so painful, I really hope Nix never do

use xxx

but directly use the items when needed.