Bug#749374: display-manager.service can't be re-enabled automatically

[Alessandro G.]

In Debian 8 "Jessie" testing 20140929, I can successfully prevent the X 
server from running automatically at boot time with:

         systemctl disable gdm

But, once disabled, it can't be re-enabled with "systemctl enable gdm" 
(or gdm3, or display-manager, or something similar).

It's necessary to manually restore the symlink erased by systemctl with:

         ln -s /lib/systemd/system/gdm.service 
/etc/systemd/system/display-manager.service

Finally, the system boots again into X/gdm3.