Bug#643634: restore nested login in gdm3

[dave bl]

Ok I have made a github repo with what should be clean patches. If the
patches aren't clean (or I have accidentally used tab instead of a
space, please let me know).

The repo is at https://github.com/d1b/gdm3 .
I'll update the upstream bug report (hoping that they will accept the
patch with ENABLE_XHOST_AUTH now surrounding the
gdm_slave_setup_xhost_auth code in gdm-slave.c.