Make it harder to push to release

[tombettany]

Extend the push command to ask for confirmation when pushing to the stable suite (similar to what apt-get install pkg does).

What needs to be done

1. Change the push subcommand here to ask for confirmation if the suite is stable.
2. Change the dr build <pkg> -p <suite> shortcut to pushing to ask for the same confirmation when pushing to stable.
3. Implement a similar confirmation for dr unpush subcommand from stable.