`SO_RCVTIMEO` is advertized but not supported

`SO_RCVTIMEO` was added by @stlankes in https://github.com/hermit-os/hermit-rs/commit/a633acd068975300313343789914a1ef8dedac84.

https://github.com/hermit-os/kernel/pull/2710 makes them known to the kernel.

This constant is used in the wild, such as urec **2** (https://github.com/hermit-os/kernel/pull/2490). To reproduce:

```toml
[package]
name = "hello_world"
edition = "2021"

[target.'cfg(target_os = "hermit")'.dependencies]
hermit = { path = "../../hermit" }

[features]
default = ["hermit/acpi", "hermit/pci", "hermit/loader", "hermit/strace", "hermit/dns"]

[dependencies]
ureq = { version = "2", default-features = false }

```

```rust
#[cfg(target_os = "hermit")]
use hermit as _;

fn main() -> Result<(), ureq::Error> {
let body: String = ureq::get("http://example.com")
.set("Example-Header", "header value")
.call()?
.into_string()?;
dbg!(body);
Ok(())
}
```

```bash
cargo xtask ci rs --arch x86_64 --package hello_world qemu --devices virtio-net-pci
```

Should we implement it or just ignore it as proposed in https://github.com/hermit-os/kernel/pull/2490?