Implement systemd socket activation

[Nhqml]

This PR implements systemd socket activation using listenfd crate.

If you're not familiar with systemd socket activation, I recommend reading these two blog posts that underline its goals and how it operates:

• https://ilmanzo.github.io/post/systemd-socket-activated-services/
• https://0pointer.de/blog/projects/socket-activation.html

You can quickly test the behaviour with these two unit files:

# bore.socket
[Unit]
Description=Bore Control Socket

[Socket]
ListenStream=7835
KeepAlive=true

[Install]
WantedBy=sockets.target

# bore.service
[Unit]
Description=Bore Server
Requires=bore.socket
After=network.target

[Service]
Type=simple
ExecStart=/usr/local/bin/bore server --min-port 5000 --max-port 6000
Restart=always

[Install]
WantedBy=multi-user.target

In the event you're interested in merging this PR, I'll be happy to contribute more detailed documentation and examples on how to integrate with systemd.