mount(2) wrapper

[cyphar]

This is a slightly harder problem than it looks, for a few reasons:

• We need to have a Rust-level API for mount(2), which is already a bit hard given that the semantics of mount(2) are incredibly odd and probably should be at least 5-6 separate methods. How those methods should be split and how we should expose very filesystem-specific features like mount options is also going to be fun (and almost every mount wrapper gets this wrong...).

• There is a new mount API (since 5.3-ish) which we should support since certain features will only be supported by it (and it also has some other "fun" features like being able to directly mount to a file descriptor without needing to go through procfs as well as mounting over symlinks :fire: :fire: :fire:). Ideally this should be compatible with the mount(2) methods (meaning we use the new mount API opportunisically) but this is going to be extra fun because fsconfig(2) is incredibly different to mount(2)-style flags and options. But maybe we should just go with the fsconfig(2) style and then mount

• It's unclear whether mounts should be an operation on a handle (which is more accurate, but has weird semantics -- like that you can umount a handle which wasn't mounted over and doesn't point at a mountpoint because in-kernel the struct path gets resolved and that's all that matters) or on a root (more in keeping with other operations but is still a little bit odd in that you can't be sure what path you're dealing with -- imagine mounting procfs inside a container).

Not to mention all of the above should be presented through C FFI in a way that makes sense.