Handbook fix: Xfce with XDM

[Davor Cubranic]

With a clean install of FreeBSD 10.1 in VirtualBox, I had to add:

DisplayManager*authName: MIT-MAGIC-COOKIE-1

to XDM config in order to get it to start xfce. This is not mentioned in the handbook, so I was getting error messages in my .xsession-errors. The fix was described in http://freebsd.1045724.n5.nabble.com/Re-ports-154502-x11-xdm-authorization-failure-when-used-with-E17-window-manager-td5819660.html