This Login.app is a simple "greeter" program you can use for XDM sessions.
It is NOT a standalone XDM-like implementation, rather it takes advantage of
the modularity of GDM. GDM use special clients (gdmlogin or gdmgreater),
called "greeter" clients, to do the graphical part of a login.
The authentication, etc., is still done by the GDM daemon.
GDM communicates through pipes and a simple protocol with the clients
(check the GDMClient class and the list of opcodes to have a better idea).