Add trait to reduce duplicated code between socket address types, implement netlink addresses

[kevinmehall]

#918

This adds a trait SockAddr, and implements it for SocketAddrV4, SocketAddrV6, SocketAddrUnix, SocketAddrXdp, SocketAddr, SocketAddrAny, and the newly-added SocketAddrNetlink. The syscalls that take a sockaddr are reimplemented in terms of this trait, removing hundreds of lines of duplicated code. It also simplifies the public interface, allowing rustix::net::{bind, connect, sendto, sendmsg_addr} to take an (addr: &impl SockAddr), rather than needing separate functions bind_v4, bind_v6, bind_unix, bind_xdp, bind_any etc. All of those are left as wrappers around the generic version, but could be deprecated and removed.

Open questions

• SocketAddrAny: So far this only changes the syscalls that take a socket address as an in-pointer. Those that return an address via an out-pointer currently return SocketAddrAny. If it is desirable to allow types outside of rustix to implement the SocketAddress trait, it would be ideal for SocketAddrAny to wrap sockaddr_storage instead of being a Rust enum. This would allow SocketAddrAny be fully generic and allow these syscalls to be used with any address type not known to rustix. Address types could use .into() / .try_into() to convert to / from SocketAddrAny. This would require a breaking change.

[sunfishcode]

As a quick update here, I am looking forward to reviewing this; I've just been busy.

[niklaswimmer]

I like this approach, it allows me to very easily define a socket address not yet present in this crate, like so

#[derive(Debug, Clone, Copy)]
#[allow(dead_code)] // fields are never read directly because they get memcp'd into another struct
#[repr(C)]
struct SocketAddressHci {
    hci_family: rustix::net::AddressFamily,
    hci_dev: u16,
    hci_channel: u16,
}

unsafe impl SocketAddress for SocketAddressHci {
    /// SAFETY: `SocketAddressHci` has the same layout as the type in the kernel and can therefore safely be passed in a syscall
    type CSockAddr = Self;

    fn encode(&self) -> Self::CSockAddr {
        self.clone()
    }
}

I left some comment about the code structure, most of it more nit than anything else. I am interested to here your opinion on using something like a Bow for the return value of encode tho. It would allow impl's to avoid the clone without having to reimplement write_sockaddr and with_sockaddr again :)

[kevinmehall]

Given that sockaddr was included on the list of libc types to remove in #1063, I slimmed down the SockAddr trait further to only with_sockaddr and made the raw pointer type an opaque struct to avoid all libc types in the API.

I also rebased to clean up the messy history for easier review. Full history archived here.