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.
add test for Basic Authentication scheme

Git-Dch: Ignore
switch tests to Translation-en usage

We can use either and some tests exercise this, but the default should
be what we want to use and that is a split out long description file
which is properly mentioned in the Release file.

Git-Dch: Ignore
tests: support 'installed' release in insertpackage

It is sometimes handy to have an installed package also in the archive,
but this was until now harder than it should as you had to duplicate the
lines, which is especially dangerous while writing the tests as it
easily happens that these two lines divert and so the same-but-different
version detection kicks in.

Git-Dch: Ignore
promote filesize to a hashstring

It is a very simple hashstring, which is why it isn't contributing to
the usability of a list of them, but it is also trivial to check and
calculate, so it doesn't hurt checking it either as it can combined even
with the simplest other hashes greatly complicate attacks on them as you
suddenly need a same-size hash collision, which is usually a lot harder
to achieve.
rewrite ReadMessages()

Central methods of our infrastructure like this one responsible for
communication with our methods shouldn't be more complicated then they
have to and not claim to have (albeit unlikely) bugs.

While I am not sure about having improved the first part, the bug is now
gone and a few explicit tests check that it stays that way, so nobody
will notice the difference (hopefully) – expect that this should a very
tiny bit faster as well as we don't manually proceed through the string.

Git-Dch: Ignore
tests: enhance output of grep and test fails

Git-Dch: Ignore
move permission changing from -item to -worker

The worker is the part closest to the methods, which will call the item
methods according to what it gets back from the methods, it is therefore
a better place to change permissions as it is very central and can do it
now at the point the item is assigned to a method rather than then it is
queued for download (and as before while dequeued via Done/Failure).

Git-Dch: Ignore
Call "Dequeue()" for items in AbortTransaction() to fix race

The pkgAcquire::Run() code works uses a while(ToFetch > 0) loop
over the items queued for fetching. This means that we need to
Deqeueue the item if we call AbortTransaction() to avoid a hang.