Revert "Do not generate an cookie as root when loading"

This reverts commit fed4519b3fa90f92cef1a4f9802eabc9692508c1.
Change authDir into stateDir
Set the auth file name to be the display name only
Change the user and group of the auth file to the sddm user when possible
Don't create .Xauthority for the sddm user
Merge pull request #180 from AnAkIn1/authfilefix

Fix cookie generation and set the correct cookie path for the greeter
Separate PamService from Authenticator

Move all PAM related code to separate files.
This reduces the code in Authenticator making it more readable and
manageable.
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.