os: add support for systemd notification

It can take some time for Xorg to start. If Xorg runs as a systemd service and other services are based on it, they have no way to determine when Xorg is really ready to accept requests. Let's use sd_notify() provided by libsystemd to signal systemd for readiness.

Signed-off-by: Topi Miettinen toiwoton@gmail.com