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.

debian/apt.postinst

@@ -43,6 +43,11 @@ case "$1" in
 	adduser --force-badname --system -home /var/empty \
 	    --no-create-home --quiet _apt || true
 
+        # ensure the passwords can still be read by the methods
+        if [ -e /etc/apt/auth.conf ]; then
+            chown _apt:root /etc/apt/auth.conf
+        fi
+
 	# deal with upgrades from experimental
 	if dpkg --compare-versions "$2" 'eq' '1.1~exp3'; then
 	    # libapt will setup partial/ at runtime