Bug#755359: [kdm] systemd seem to fail to start display-manager.service

[Francesco Muzio <muziofg@gmail.com>]

also I think it's impossible that a soft-link 
/etc/systemd/system/display-manager.service is created on a fresh 
install who only installs KDM as display-manager.service.

Otherwise on the system there would be a 
/etc/systemd/system/display-manager.service who not point to no service.