chown finished partial files earlier

chown finished partial files earlier

partial files are chowned by the Item baseclass to let the methods work
with them. Now, this baseclass is also responsible for chowning the
files back to root instead of having various deeper levels do this.

The consequence is that all overloaded Failed() methods now call the
Item::Failed base as their first step. The same is done for Done().

The effect is that even in partial files usually don't belong to
_apt anymore, helping sneakernets and reducing possibilities of a bad
method modifying files not belonging to them.

The change is supported by the framework not only supporting being run
as root, but with proper permission management, too, so that privilege
dropping can be tested with them.