Attempt to notify systemd of service readiness on linux

[nbrownus]

This will send READY=1 to the unix domain socket described by the NOTIFY_SOCKET environment variable, if it was provided, after we have called Control.Activate().

Should resolve timing issues with services that depend on nebulas udp listener(s) and tun device being up.

Closes #921

[johnmaguire]

This looks correct to me.

[jdoss]

LGTM!

[krabiswabbie]

Hello,

I wanted to report an ongoing issue with the example systemd service description provided in examples/service_scripts. Despite the feature being marked as resolved, I've found that Nebula fails to notify systemd of its successful startup when the systemd service is configured with Type=notify. As a result, the Nebula service continuously restarts due to a timeout.

This behavior is observed in the default installation of v1.7.2 on both amd64 and arm64 architectures. I'm aware that switching the systemd service type to simple is a workaround, but thought it would be beneficial for the team to know that the issue persists, especially considering it might affect new users trying to set up Nebula using the example provided.

Thank you for your dedication to this project.

[johnmaguire]

Hi @krabiswabbie - this PR has not made it into a release yet. You can try running one of the nightly builds now if you like: https://github.com/NebulaOSS/nebula-nightly/releases