l10n: de.po: fix some minor issues

This fixes some minor issues and improves the
German translation a bit. The following things
were changed:
- use complete sentences in option related messages
- translate "use" consistently as "verwendet"
- don't translate "make sense" as "macht Sinn"

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Acked-by: Thomas Rast <trast@inf.ethz.ch>