OK, I did a fresh install and found something that I had missed. Namely that the shutdown options were also non functional in the lightdm boot screen, so it appears that the problem is more likely in the autoboot install of lightdm. Just to verify what is happening I did another fresh install but installed mate desktop using the dietpi-software option. Then did the dietpi-autoboot 16 thing. Rebooted and the shutdown options worked just fine in both the lightdm boot screen and in mate. So I uninstalled the mate desktop using dietpi-software. Then I installed cinnimon-core and the shutdown options are now working in both the lightdm boot screen and in cinnamon. So it would appear that something is missing when you do the dietpi-autoboot 16 command before installing cinnmon-core. Oh well, I got the desktop I wanted up and running even if it did take a few strange steps to get it working.