QAuth: Don't start session command for the greeter session

Greeter must be launched as is.
Add information about setting up the sddm user

With pam and systemd, the greeter will go through logind which will give
it acces to drm devices.

Systems without pam and systemd will need to put the sddm user into
the video group, otherwise they might experience errors accessing
the devices.  See issue #184.

Inform our users about the fact that we now always need the "sddm"
user.