Fix kscreenlocker_greet --switchuser
Years ago support was added for the lockscreen to default into switch user
mode. The goal being to simplify one more task from ksmserver.
It got postponed because it clashed with a change Kai made at the time.
Now I'm restoring that.
The code broke in the lockscreen rewrite.
Called the switchUser DBus method
Lockscreen spawned on the right page
Locked screen normally, got the normal page
Reviewers: Plasma, mart
Reviewed By: Plasma, mart
Differential Revision: https://phabricator.kde.org/D15185