use listen-fd crate

[yoshuawuyts]

https://github.com/mitsuhiko/rust-listenfd

Apparently some bookkeeping is required to correctly use the Systemd LISTEN_FD sockets. It might be useful for our .bind() and .bind_or() methods to make use of this.

[yoshuawuyts]

Looks like our current implementation is broken tbh. We should be listenening for both LISTEN_FDS and SYSTEMFD_SOCKET_SERVER

• https://github.com/mitsuhiko/rust-listenfd/blob/master/src/windows.rs#L69
• https://github.com/mitsuhiko/rust-listenfd/blob/master/src/unix.rs#L78

Perhaps a conditional compile would work best here?

I'm thinking we should detect if either value is set, and then let rust-listenfd handle it.