`SO_RCVTIMEO` is advertized but not supported

[mkroening]

SO_RCVTIMEO was added by @stlankes in hermit-os/hermit-rs@a633acd.

#2710 makes them known to the kernel.

This constant is used in the wild, such as urec 2 (#2490). To reproduce:

[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 }

#[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(())
}

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 #2490?