client: Use the default-user gsetting value added to unity-greeter
to set the primary user as the default user when lightdm starts.
Also set greeter-hide-users=false in /etc/lightdm/lightdm.conf
when primary user is set.
client: Set exclude-prefixes to filter out admin users.
Merge remote-tracking branch 'origin/trusty.unity-greeter' into trusty