Support processing ancillary outside rustix infrastructure

[SUPERCILEX]

Specifically, I'm trying to parse control data from io_uring responses.

[sunfishcode]

Looks good. Two observations:

Looking at it now, it looks like a mistake to have AncillaryDrain hold read and length members. Rust standard library things like Vec::drain remove their elements even if the iterator is dropped without being iterated, so we should ideally do the same, and if we did, that would also eliminate this Option. However, we can do that in the next semver bump, because it is a subtle behavior change.

In general, it's a goal of rustix to provide safe APIs wherever possible, so this unsafe parse function is unforuntate, however a safe RecvMsg API for io_uring would be non-trivial, and possibly the best way thing to do within rustix is to just expose the unsafe primitives and look to higher-level crates to provide the safe abstractions on top of them.