Autologin greeter for LightDM

 Minimal autologin greeter for LightDM that has the same auto-login
 behaviour as nodm, but being based on LightDM it stays on top of modern
 display manager requirements.
 .
 The difference between LightDM's built-in auto-login and this greeter,
 are the behaviour in case of 0-seconds autologin delay: when LightDM
 autologs in with no delay, upon logout it will show the login window
 again. The intent is that, if the default user logged out, they probably
 intend to log in again as a different user.
 .
 When managing a kiosk-like setups, once the X session quits, the
 desired behaviour is to just start it again.
 .
 LightDM with an auto-login timeout of 1 or more seconds would work as
 needed, but one sees the login dialog window appear and disappear on
 screen at each system startup. While it is functional, on a kiosk setup
 this is aesthetically unprofessional.
 .
 With this greeter, the X session starts right away, and is restarted if
 it quits, without any flicker of a login dialog.
 .
 If one is not setting up a kiosk-like setup, it's very likely that the
 default auto-login behaviour of LightDM is the way to go, and that this
 greeter is not needed.