Use Rust stdlib Unix socket

[curiousleo]

rust-lang/rust#14194 has been resolved, unix_socket is deprecated.

[curiousleo]

This null byte

rust/varlink/src/server.rs

Line 147 in 52433a0

addr = addr.replacen('@', "\0", 1);

appears to make the stdlib socket implementation unhappy: https://travis-ci.org/varlink/rust/jobs/622583955#L348

So this needs more work.

[curiousleo]

unix_socket is unmaintained, but rust-lang/rust#14194 was closed as "won't fix", not as "done". So this isn't going to work.

[haraldh]

maybe the nix crate can help