Use sockets passed in from systemd

[flokli]

This would allow yubikey-agent to be started socket-activated by systemd.
Background: https://vincent.bernat.ch/en/blog/2018-systemd-golang-socket-activation

[tv42]

Exiting on idle and letting socket activation restart the service would make #4 slightly less likely to bother me, too.

Yes, that would forget the PIN when idle, but that might even be desirable...

Not sure if there's (still) a race between shutting down and getting new connections, AFAIK that was not easy to handle right some 5 years back.

[FiloSottile]

I would take a PR for this, but can't implement it myself as I don't regularly use Linux desktop environments.