l10n: de.po: address the user formally

In the current German translation, the user was
addressed informally ("Du", "Dein") which is unusual
in German software. This commit changes the addressing
to be formal ("Sie", "Ihr").

Suggested-by: Christian Stimming <stimming@tuhh.de>
Suggested-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>