Open a PAM session for the greeter

This allows the greeter to pass through logind which will pass the devices
to the greeter.

Closes #184.
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 system will need to put the sddm user into
the video group, otherwise they might experience errors accessing
the devices.  See issue #184.