It looks like in a recent update, lightdm (with the gtk interface, at least) lost some buttons for rebooting or shutting down the computer. Which is somewhat cumbersome, as one has to Ctrl-Alt-F1 his way to a console to actually do the shutdown manually.

This is described e.g. in the following bug reports with a possible solution: here or here

The solution is simply to edit /etc/lightdm/lightdm-gtk-greeter.conf and replace the #indicators= line with indicators=~clock;~spacer;~host;~spacer;~language;~session;~power.