agent: broadcast to consoles before rebooting

[lucab]

This is a followup to coreos/fedora-coreos-tracker#239.

Right now Zincati triggers a reboot in a silent way, which could surprise any logged users while they are in middle of some interactive maintenance.

For better user experience, it should somehow notify the users before triggering a reboot. Current proposal is to broadcast (i.e. wall) to all active consoles. This would match locksmith behavior.

[lucab]

Blocked until:

• utmpx: start adding accounting-related POSIX functions nix-rust/nix#1111 is fixed
• a new version of nix crate is published with the fix

[dustymabe]

+1 - I just had this idea today and there's already an open issue for it ❤️

[lucab]

Small update here, this is unfortunately blocked on a couple more levels of dependencies that need to be patched too, see rust-lang/libc#1496 (comment).

[kelvinfan001]

Update: The implementation in #479 simply shells out to loginctl to get the list of logged in users. No longer requiring nix-rust/nix#1111.