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.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.