[Merge] lp:~mvo/update-manager/use-screen-in-text-frontend into lp:update-manager

[Michael Vogt]

This branch will automatically use gnu screen if available to run the release upgrader inside a screen window called "ubuntu-release-upgrade-screen-window". If the terminal
goes away for some reason (e.g. network disconnect because of network problem or upgrade 
problem) re-connecting and re-running the upgrade command will resume the upgrade.

For a admin running inside screen already nothing should change (except for the name of
the screen window) the command key will work normally. For a admin not using screen the
command key is disabled to avoid confusion with e.g. people used to emacs ctrl-a.

Zombie mode is enabled so that if the upgrade exits the screen content is still visible
for the admin to inspect.

Please let me know if there are any side effects I have not considered.
-- 
https://code.launchpad.net/~mvo/update-manager/use-screen-in-text-frontend/+merge/42114
Your team Ubuntu Core Development Team is requested to review the proposed merge of lp:~mvo/update-manager/use-screen-in-text-frontend into lp:update-manager.