(Maint) Deprecate the `prompt` parameter

The `prompt` parameter was intended to allow a reboot to be canceled
when running puppet interactively, e.g. for testing, demo, etc. However,
if you are running `puppet agent`, and prompt is true, the system will
display a dialog box on the hidden service desktop (since services are no
longer allowed to interact with the desktop). This will cause the agent to
hang waiting for user input that never arrives.

This commit causes puppet to issue a deprecation warning in the case that
prompt is set to true.

Instead of relying on the prompt parameter, instead issue the command:

    shutdown /a

to abort a shutdown during testing, etc.