Auto merge of #85379 - mdaverde:uds-abstract, r=joshtriplett

Add abstract namespace support for Unix domain sockets

Hello! The other day I wanted to mess around with UDS in Rust and found that abstract namespaces ([unix(7)](https://man7.org/linux/man-pages/man7/unix.7.html)) on Linux still needed development. I took the approach of adding `_addr` specific public functions to reduce conflicts.

Feature name: `unix_socket_abstract`
Tracking issue: #85410
Further context: #42048

## Non-platform specific additions

`UnixListener::bind_addr(&SocketAddr) -> Result<UnixListener>`

`UnixStream::connect_addr(&SocketAddr) -> Result<()>`

`UnixDatagram::bind_addr(&SocketAddr) -> Result<UnixDatagram>`

`UnixDatagram::connect_addr(&SocketAddr) -> Result<()>`

`UnixDatagram::send_to_addr(&self, &[u8], &SocketAddr) -> Result<usize>`

## Platform-specific (Linux) additions

`SocketAddr::from_abstract_namespace(&[u8]) -> SocketAddr`

`SockerAddr::as_abstract_namespace() -> Option<&[u8]>`

## Example

```rust
#![feature(unix_socket_abstract)]
use std::os::unix::net::{UnixListener, SocketAddr};

fn main() -> std::io::Result<()> {
    let addr = SocketAddr::from_abstract_namespace(b"namespace")?; // Linux only
    let listener = match UnixListener::bind_addr(&addr) {
        Ok(sock) => sock,
        Err(err) => {
            println!("Couldn't bind: {:?}", err);
            return Err(err);
        }
    };
    Ok(())
}
```

## Further Details

The main inspiration for the implementation came from the [nix-rust](https://github.com/nix-rust/nix/blob/master/src/sys/socket/addr.rs#L558) crate but there are also other [historical](https://github.com/rust-lang/rust/commit/c4db0685b181f12c4285dac3d932f1859bba74f5) [attempts](https://github.com/tormol/uds/blob/master/src/addr.rs#L324) with similar approaches.

A comment I did have was with this change, we now allow a `SocketAddr` to be constructed explicitly rather than just used almost as a handle for the return of `peer_addr` and `local_addr`. We could consider adding other explicit constructors (e.g. `SocketAddr::from_pathname`, `SockerAddr::from_unnamed`).

Cheers!

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