Don't pad vt number with zeros

It's not required, plus other display managers don't pad with zeros.
Make the greeter go through logind

Use QAuth for the greeter session so that it will be registered
as a logind session with class "greeter".

With this, logind can pass drm devices to the greeter,
resolving all permission denied errors.

This work will also be needed by the Wayland port.

From now on we don't support running the greeter as root,
however the old code is preserved for test mode.
QAuth: Don't override XAUTHORITY every time

Setting XAUTHORITY makes sense only if DISPLAY is not and we don't want
to override any previous value to preserve what greeter set.
QAuth: Use sddm-greeter service for the sddm user

Use the sddm-greeter PAM service if we are authenticating the sddm
user.
QAuth: Pass process environment to PAM

This will make logind happy as it will know about seat, vt, session
type and session class.
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.
Merge pull request #192 from plfiorini/greetersessionqauth

Greeter session