rust/chg/build.rs

rust-chg: add function to send fds via domain socket

As a beginning, I wrote some C.

It's extracted from attachio() of contrib/chg/hgclient.c. Maybe it could
be rewritten in Rust by using the libc (and/or nix) crates, but doing that
wouldn't be trivial as the code depends on CMSG_*() macros. IMO, using C
is better here.

extern crate cc;

fn main() {
    cc::Build::new()
        .warnings(true)
        .file("src/sendfds.c")
        .compile("procutil");
}