Clean fsopen a bit & add support for mount_setattr

[livingsilver94]

This is my attempt at supporting mount_setattr, taking into consideration the fact struct mount_attr is designed to be expandable, and that it currently borrows a user namespace's file descriptor. See discussions in PR #1002 for more details.

The idea is to have a MountAttr struct that is a thin wrapper around struct mount_attr, that also manages lifetimes. The
underlying type is not exposed: you can only set parameters via the constructor and setters.

While at it, I cleaned fsopen (and some related modules) a little. See the commit history.

Fixes #975.

Test plan

[package]
name = "test"
version = "1.0.0"
rust-version = "1.63"
edition = "2021"

[dependencies]
rustix = { path = "/your/local/fork/of/rustix", features = [
    "mount",
    "linux_latest", # Or "use-libc" to test the other backend.
    "fs",
] }

use std::os::fd::AsFd;

use rustix::fs::{open, Mode, OFlags};
use rustix::io;
use rustix::mount::{
    mount_setattr, MountAttr, MountAttrFlags, MountPropagationFlags, MountSetAttrFlags,
};

// To create a test mount point, run:
// $ mkdir /tmp/testpoint
// # mount -t tmpfs tmpfs /tmp/testpoint

fn main() -> io::Result<()> {
    let dirfd = open("/tmp", OFlags::RDONLY | OFlags::DIRECTORY, Mode::empty())?;

    // Uncomment to test set_param_flag.
    // mount_setattr will return EINVAL, because this won't be a user namespace fd.
    // let fd = open("/dev/null", OFlags::RDONLY, Mode::empty())?;

    let attr = MountAttr::new(
        MountAttrFlags::MOUNT_ATTR_RDONLY,
        MountAttrFlags::empty(),
        MountPropagationFlags::empty(),
    );

    // Uncomment to test set_param_flag.
    // let attr = attr.set_param_flag(rustix::mount::MountAttrParamFlags::IdMap(fd.as_fd()));

    // Uncommenting this line gives a compilation error when set_param_flag is called.
    // drop(fd);

    mount_setattr(dirfd, "testpoint", MountSetAttrFlags::AT_RECURSIVE, attr)?;

    println!("/tmp/testpoint is now mounted read-only");
    Ok(())
}

[livingsilver94]

Converting to draft as the CI is returning errors I did not have on my system running Fedora + Rust 1.96.

[livingsilver94]

Done. libc is not compatible with Rust 1.63 and it's making the CI fail, but this is outside of the scope of this PR unless you want me to fix that.