This implements socket activation for radicle-node by cooperating with systemd. It is an alternative to patch b25bed2, which uses libsystemd to achieve the same goal. Only one of the two patches should be accepted.

A recording of socket activation in action, where execution of rad node status automatically starts the node: https://asciinema.org/a/R7N2N9JPvU8MMQh4LRD2FJQzW.

Note that socket activation is nothing specific to systemd. On OS X, launchd supports the same mechanism with a similar API. The JavaScript library node-socket-activation implements socket activation for both systemd and launchd.

See also:

• man:sd_listen_fds(3)
• Crate libsystemd
• Crate raunch (analogue for launchd)
• https://0pointer.net/blog/projects/systemd.html
• https://0pointer.de/blog/projects/socket-activation.html
• https://0pointer.de/blog/projects/socket-activation2.html
• https://github.com/moby/moby/pull/3105
• https://github.com/sorccu/node-socket-activation
• https://mgdm.net/weblog/systemd-socket-activation/

Rebase.