remove useless pdiff filename output

Looks like a leftover from debugging. Absolutely no need for it and
destroys progess reporting completely.

Closes: 764737
make --allow-insecure-repositories message an error

Not using this option, but using unsigned (and co) repositories will
cause these repositories to be ignored and data acquiring from them
fails, so this is very well in the realms of an error and helps in
making 'apt-get update' fail with a non-zero error code as well.
display a warning for unsigned repos

The same message is used for InRelease if fails in gpgv, but the
Release/Release.gpg duo needs to handle the failing download case as
well (InRelease just defers to the duo if download fails) and print a
message accompaning the insecure error to provide a hint on what is
going on.
do not inline virtual destructors with d-pointers

Reimplementing an inline method is opening a can of worms we don't want
to open if we ever want to us a d-pointer in those classes, so we do the
only thing which can save us from hell: move the destructors into the cc
sources and we are good.

Technically not an ABI break as the methods inline or not do the same
(nothing), so a program compiled against the old version still works
with the new version (beside that this version is still in experimental,
so nothing really has been build against this library anyway).

Git-Dch: Ignore
trusted=yes sources are secure, we just don't know why

Do not require a special flag to be present to update trusted=yes
sources as this flag in the sources.list is obviously special enough.

Note that this is just disabling the error message, the user will still
be warned about all the (possible) failures the repository generated, it
is just triggering the acceptance of the warnings on a source-by-source
level.

Similarily, the trusted=no flag doesn't require the user to pass
additional flags to update, if the repository looks fine in the view of
apt it will update just fine. The unauthenticated warnings will "just" be
presented then the data is used.

In case you wonder: Both was the behavior in previous versions, too.
Document Acquire{MaxReleaseFileSize,AllowInsecureRepositories,AllowDowngradeToInsecureRepositories} and --no-allow-insecure-repositories

Document the new options to restrict loading unauthenticated data
into our parsers.
Fix backward compatiblity of the new pkgAcquireMethod::DropPrivsOrDie()

Do not drop privileges in the methods when using a older version of
libapt that does not support the chown magic in partial/ yet. To
do this DropPrivileges() now will ignore a empty Apt::Sandbox::User.

Cleanup all hardcoded _apt along the way.