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.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/chg/build.rs	Mon Sep 24 16:14:35 2018 +0900
@@ -0,0 +1,8 @@
+extern crate cc;
+
+fn main() {
+    cc::Build::new()
+        .warnings(true)
+        .file("src/sendfds.c")
+        .compile("procutil");
+}