Notify service manager when start up is completed

[one-d-wide]

Currently there is no way to notify service manager when yggdrasil start up process is truly completed, i.e its TUN interface and admin API endpoints are up and running. This makes naive services, set to depend on the yggdrasil network, periodically crash on startup, requiring sub-optimal complications for trivial tasks.

This PR solves the issue by notifying the service manager after TUN interface and admin sockets are ready. Systemd support is implemented by sending a short READY=1 message to a UNIX datagram socket defined in a NOTIFY_SOCKET environment variable, see sd_notify(3).

Changes

• cmd/yggdrasil/main.go - Added invocation for notifyStartupCompleted right after TUN interface and admin sockets are set up.
• cmd/yggdrasil/notify_startup_*.go - Added notifyStartupCompleted function handling the process on linux and immediately returning on other platforms.
• contrib/systemd/yggdrasil.service* - Service Type is changed to notify, added a comment about NotifyAccess property.