usetup: German translation revamp

some of the most common changes:

• "Enter" > "Eingabetaste" (as it is used in WinXP), "Boot-Loader">"Bootloader", ...
• tried to avoid usage of "Setup" (this is actually not German, but Windows uses it anyway),
  mostly by using the passive form (e.g. instead of "Setup hat den Fehler XXX festgestellt" "Fehler XXX wurde festgestellt")
• consistent use of "neu starten" instead of "neustarten" (Duden recommends it)

I attach some screenshots (VBox), because the patch has a pretty strange character encoding.

Please, share your suggestions about my changes so that we can improve them when needed.