examples: Add systemd startup script and example session
This allows running a session using a separate systemd user instance. The advantage is full separation of the normal user (e.g. to run a nested session) and generally a better flexibility and more straight-forward way of pulling in systemd units from jhbuild.
wip/benzea/systemd-user-session in 14 minutes and 40 seconds (queued for 1 second)2 jobs for