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.
#require serve
$ hg clone http://localhost:$HGPORT/ copy
abort: * (glob)
[255]
$ test -d copy
[1]
$ "$PYTHON" "$TESTDIR/dumbhttp.py" -p $HGPORT --pid dumb.pid
$ cat dumb.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/foo copy2
abort: HTTP Error 404: * (glob)
[255]
$ killdaemons.py