sharing: Use systemd to track running services

Instead of launching network services by hand, and having no way to
track them if gnome-settings-daemon crashes, leaving the services
dangling open when they should have been switched off, use systemd
--user to track the services.

This means that:
- services will not be left on when switching to a network where they
  should be disabled
- services will be restarted if they crash

https://bugzilla.gnome.org/show_bug.cgi?id=766329