[debian] don't require (fake)root to run the clean target. Closes #3161797 It works just fine as a normal user.

[debian] Add ${misc:Depends} to cube packaging skeleton. Closes #3161797 Best current practice is to have that in Depends in all packages using debhelper.

[devtools] fix race when creating backupdir If somebody else creates backupdir between our stat() and mkdir() (like, say, another test running in parallel), we shouldn't crash.