Support for autostarting using systemd socket activation

[jonnor]

One generally has several projects made with NoFlo. And when switching between them in Flowhub, one needs to set up the corresponding runtime (usually node-nodejs) in the right directory, before one can connect to it.
However, with systemd socket activation a service can be started (and then stopped after a time) when traffic comes in on a port, for instance from Flowhub.
http://0pointer.de/blog/projects/socket-activation.html

systemd provides some C APIs for this, accessible for instance via https://github.com/rubenv/node-systemd
However, it sounds like that may not be neccesary, as systemd also sets LISTEN_FDS envvar, and then file descriptor 3 is the one to use. rubenv/node-systemd#12 (comment)

[jonnor]

Might also want a systemd .service unit generator, like msgflo/msgflo#20