Feature Request: Add support for `IP_HDRINCL` socket option

### Problem I am currently working with raw sockets and need to set the `IP_HDRINCL` option to manually construct IP headers. Currently, `rustix::net::sockopt` does not appear to expose a wrapper for this option. As a result, I have to fall back to `unsafe` code and `libc` to achieve this, which breaks the flow of using `rustix` for safe system calls. ### Current Workaround Currently, I have to implement it like this: ```rust use rustix::fd::AsRawFd; let one: i32 = 1; unsafe { libc::setsockopt( fd.as_raw_fd(), libc::IPPROTO_IP, libc::IP_HDRINCL, std::ptr::from_ref::<i32>(&one).cast(), std::mem::size_of::<i32>() as u32, ); } ``` ### Proposed Solution It would be great to add `set_ip_hdrincl` (and potentially `get_ip_hdrincl`) to `rustix::net::sockopt`. **Desired API:** ```rust // In rustix::net::sockopt pub fn set_ip_hdrincl<Fd: AsFd>(fd: &Fd, value: bool) -> rustix::io::Result<()> { // ... implementation wrapping setsockopt with IPPROTO_IP / IP_HDRINCL } pub fn get_ip_hdrincl<Fd: AsFd>(fd: &Fd) -> rustix::io::Result<bool> { // ... implementation wrapping getsockopt } ``` ### Context This is standard functionality for raw socket programming (e.g., when building custom network tools) and seems to fit well within the scope of `rustix`'s networking primitives.