[Gentoo] Login function by using lightdm

1. Install package
   emerge --ask x11-misc/lightdm
   
2. Set the environment settings
   Set LightDM as the default display manager:
   # vim /etc/conf.d/xdm
   DISPLAYMANAGER=”lightdm”
   
   
   Create file .xinitrc under username folder.
   # vim ~/.xinitrc
   exec cinnamon-session
   
3. Boot service (Auto start)
   Under OpenRC
   
   # sudo rc-update add dbus default
   # sudo rc-update add xdm default
   Must use sudo command otherwise the "rc-update" won't appear.
   
   Reference:
   
   • LightDM
   • lightdm fails to start session after authenticating