Fix session startup with new systemd, using user@.service mechanism

With these patches, we use the new systemd's user@.service to start user services. To this end, start-user-session service becomes a service which logs into 'nemo' user, and user@.service is automatically started by systemd. Boot state setup has become a separate service since start-user-session is now started as nemo user rather than root.

Finally, nemo user is added to users group because battery status information in statefs in only accessible to root and members of users group.