Initial basic cmsg support for unix

[brunowonka]

Also added set_recv_ttl while I'm here to use in testing. The other RECVxxx-type socket options might make more sense with control message support now.

I went back and forth on the shape of the public API for this quite a bit, and I also don't have access to a Windows machine to check.

Fixes #313

[Thomasdezeeuw]

I'm afraid this doesn't work at all on a number of platform, please see the CI failure. I'll do another round of review once those are fixed.

[Thomasdezeeuw]

Let's remove the generic parameter, we only need &[u8], same for the other structures.

[brunowonka]

Tentatively keeping this parameter so we can reuse some logic between &[u8] and &mut [u8], but removed all the type parameters from the public API for now.

[brunowonka]

Thank you for looking, I'm working through the other platform problems here and I'll push as soon as I have things working so you can look through.

[Thomasdezeeuw]

Not a complete review, but a start.

[darconeous]

Looking at this failure:

Run cargo install cargo-hack
    Updating crates.io index
 Downloading crates ...
  Downloaded cargo-hack v0.5.15
error: failed to parse manifest at `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec[8](https://github.com/rust-lang/socket2/runs/7569907645?check_suite_focus=true#step:4:9)23/cargo-hack-0.5.15/Cargo.toml`

Caused by:
  failed to parse the `edition` key

Caused by:
  this version of Cargo is older than the `2021` edition, and only supports `2015` and `2018` editions.
Error: Process completed with exit code [10](https://github.com/rust-lang/socket2/runs/7569907645?check_suite_focus=true#step:4:11)1.

Looks unrelated to this change?

The macOS failure looks real though:

running 36 tests
...snip...
test send_from_recv_to_vectored ... FAILED
test send_recv_vectored ... FAILED
...snip...

failures:
error: test failed, to rerun pass '--test socket'

error: process didn't exit successfully: `/Users/runner/.rustup/toolchains/stable-x86_64-apple-darwin/bin/cargo test --manifest-path Cargo.toml --no-default-features` (exit status: 101)
---- send_from_recv_to_vectored stdout ----
thread 'send_from_recv_to_vectored' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 22, kind: InvalidInput, message: "Invalid argument" }', tests/socket.rs:[57](https://github.com/rust-lang/socket2/runs/7569907933?check_suite_focus=true#step:5:58)4:10