ExitPrompter: Add a 'Show Standby' option

This adds a 'Show Standby' option to the 'Customise exit menu option' settings
which if selected will show just the 'Enter Standby Mode' on both the exit
prompter and system menus.

This is useful if you are using a combined FE/BE system and you don't want
your users to be able to shutdown or reboot the system or exit the FE.