Use QAuth for authentication

Now that SDDM includes QAuth we can take advantage of its improved
authentication support.

This is based on #151 by Martin Bříza <mbriza@redhat.com>
Improve greeter stderr and stdout logging from daemon

Unquote output and avoid extra spaces.
Also, remove curly braces from code.
Use sddm PAM service in QAuth's PAM backend

Use our naming for PAM services.
Keep in mind that we don't have the sddm-check PAM service yet.
Use message handler for sddm-helper

Log just like sddm-daemon and sddm-greeter.
Also remove redundant parts from the log messages.