Updated German version of OpenPGP/GnuPG Best Practices

Took me a while, but I brought the German version of the OpenPGP/GnuPG Best Practices up-to-date with the English version. While doing that I also changed the section on setting your key preferences (generating a new key is not necessary to do this) and fixed some minor stylistic issues (typos, spacing etc.) in the English original.
Unfortunately, during the long time it took me to do this, other people made changes and the best-practices dir has been renamed gpg-best-prafctices (good move), which made a merge necessary.

I shall endeavour to commit quicker and earlier in future.