Landlock support

Having searched through issues and PRs I have not seen any direct mentions of adding support for [Landlock](https://landlock.io) - the new-ish userspace-sandboxing syscalls initially added in the Linux kernel version 5.13, so here is that mention.

Documentation for the kernel feature is available [here](https://docs.kernel.org/userspace-api/landlock.html).

There is already a [crate](https://lib.rs/crates/landlock) made by the original author, which is actively maintaining it. However, it directly uses libc for the syscalls, thus is not purely written in Rust, which is a shame.

Is there interest in adding support for the landlock syscalls directly in rustix or should one use the official landlock crate instead and have to live with the libc dependency?