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.
fix compile and tests error

I am pretty sure I did that before committing broken stuff…

Git-Dch: Ignore
do not load filesize in pkgAcqIndexTrans explicitly

The constructor is calling the baseclass pkgAcqIndex which does this
already – and also does it correctly for compressed files which would
overwise lead to the size of uncompressed files to be expected.

Git-Dch: Ignore
update symbols file

Git-Dch: Ignore