lightdm通过systemd控制开机自启动

关闭开机自启动：

#sudo systemctl disable lightdm.service

Synchronizing state of lightdm.service with SysV service script with /lib/systemd/systemd-sysv-install.

Executing: /lib/systemd/systemd-sysv-install disable lightdm

Removed /etc/systemd/system/display-manager.service.

添加自动启动：

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

手动启动：

sudo service lightdm start