don't drop privileges if _apt has not enough rights

Privilege dropping breaks download/source/changelog commands as they
require the _apt user to have write permissions in the current directory,
which is e.g. the case in /tmp, but not in /root, so we disable the
privilege dropping if we deal with such a directory based on idea and
code by Michael Vogt.

The alternative would be to download always to a temp directory and move
it then done, but this breaks partial file support. To resolve this, we
could move to one of our partial/ directories, but this would require a
lock which would block root from using two of these commands in
parallel. As both seems unacceptable we instead let the user choose what
to do: Either a directory is setupped for _apt, downloading as root is
accepted or – which is potentially even better – an unprivileged user is
used for the commands.

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