Do not check for updates if a reboot is required from previous update run

[eloquence]

Currently, if the user starts the updater a second time, and reboots are required, it will happily run through the whole "checking for updates" process again before letting the user know that they have to reboot. Instead, we should immediately alert the user that a reboot is required to apply available updates.

The screen for successful updates that require a reboot currently looks like this:

[====100%====]
All updates complete!
A reboot is required for security updates to be applied.
Please click the Reboot button to continue.
[Reboot]

I think if the user launches the updater again now without having rebooted, it should immediately show a screen similar to the above, with two differences:

• omit the progress bar
• instead of "All updates complete", say something like "One more step" or omit the text (can kick this around a bit on this issue).

[eloquence]

(First noted here.)

[eloquence]

Tracking as stretch goal since it's an annoying waste of the user's time in a secondary path. The "happy path" is that the user immediately requests a reboot.

[zenmonkeykstop]

This should be obsoleted by coming changes in the Qubes Updater, will close it then.