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.
If Xorg has not been started as a systemd service, this won't do
anything.

Signed-off-by: Topi Miettinen <toiwoton@gmail.com>
3 jobs for sd-notify in 7 minutes and 36 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #285539
debian-testing

00:00:27

 
  Build And Test
passed #285540
autotools-build-and-test

00:07:08

passed #285541
meson-build-and-test

00:02:41