darwin: add in6_aliasreq, ifaliasreq structs

Structs such as in6_aliasreq, ifaliasreq are missing in libc. We should consider adding them.

Custom bindings that I use right now.

#[allow(non_camel_case_types)]
#[repr(C)]
#[derive(Copy, Clone)]
pub struct in6_aliasreq {
    pub ifra_name: [c_char; IFNAMSIZ],
    pub ifra_addr: sockaddr_in6,
    pub ifra_dstaddr: sockaddr_in6,
    pub ifra_prefixmask: sockaddr_in6,
    pub ifra_flags: c_int,
    pub ifra_lifetime: in6_addrlifetime,
}

#[allow(non_camel_case_types)]
#[repr(C)]
#[derive(Copy, Clone)]
pub struct ifaliasreq {
    pub ifra_name: [c_char; IFNAMSIZ],
    pub ifra_addr: sockaddr,
    pub ifra_broadaddr: sockaddr,
    pub ifra_mask: sockaddr,
}

Compare the XNU definitions in bsd/netinet6/in6_var.h and bsd/net/if.h with the custom Rust bindings shown in the issue, then inspect the existing Darwin libc bindings for placement and conventions. Done means both structs are exposed with the required C-compatible fields and types, with the relevant bindings checks passing.