agent: Add run_listener to the Agent trait

[wiktor-k]

This change exposes Agent::run_listener that takes a raw UnixListener object.

This allows starting the agent with listeners that are backed by raw file descriptors. This in turn makes writing agents that are started with systemd's socket activation.

The following example starts the agent from systemd's supplied input:

use std::os::unix::io::FromRawFd;

let fds = std::env::var("LISTEN_FDS").unwrap();
let fds: i32 = fds.parse()?;
let listener = unsafe { std::os::unix::net::UnixListener::from_raw_fd(fds + 2) };
listener.set_nonblocking(true)?;

let handle = tokio_reactor::Handle::default();
let listener = tokio_uds::UnixListener::from_std(listener, &handle)?;

let agent = Backend::default();
agent.run_listener(listener);