debian/apt.postinst: chown _apt:root /etc/apt/auth.conf

If the methods drop privileges we need to ensure that
/etc/apt/apt.conf is still readable by the _apt user.
Ensure /etc/apt/auth.conf has _apt:root owner

Ensure in SetupAPTPartialDirectory() that the /etc/apt/auth.conf file
can be read by the priv sep apt methods.
check that auth.conf exists before chowning it

Git-Dch: Ignore